Possible fix for X509 compilation errors for 5.15.x versions

[MichaelColonel]

Dear developers,

Linux distribution with: Qt-5.15.2, GCC-10.3, OpenSSL-1.1.1m

I usually use 5.12.2 version of OPC UA. I have a compilation error in more recent versions (with openssl support ).

In file included from x509/openssl_symbols.cpp:57:
x509/openssl_symbols.cpp: In function 'int q_BN_set_word(const BIGNUM*, long unsigned int)':
x509/openssl_symbols.cpp:276:48: error: invalid conversion from 'const BIGNUM*' {aka 'const bignum_st*'} to 'BIGNUM*' {aka 'bignum_st*'} [-fpermissive]
  276 | DEFINEFUNC2(int, BN_set_word, const BIGNUM *a, a, BN_ULONG w, w, return 0, return)
      |                                                ^
      |                                                |
      |                                                const BIGNUM* {aka const bignum_st*}
x509/openssl_symbols_p.h:217:45: note: in definition of macro 'DEFINEFUNC2'
  217 |     ret q_##func(arg1, arg2) { funcret func(a, b); }
      |                                             ^
In file included from /usr/include/openssl/asn1.h:23,
                 from x509/openssl_symbols_p.h:59,
                 from x509/openssl_symbols.cpp:57:
/usr/include/openssl/bn.h:271:25: note:   initializing argument 1 of 'int BN_set_word(BIGNUM*, long unsigned int)'
  271 | int BN_set_word(BIGNUM *a, BN_ULONG w);
      |                 ~~~~~~~~^
In file included from x509/openssl_symbols.cpp:57:
x509/openssl_symbols.cpp: In function 'int q_PEM_write_bio_PKCS8PrivateKey(BIO*, EVP_PKEY*, const EVP_CIPHER*, unsigned char*, int, int (*)(char*, int, int, void*), void*)':
x509/openssl_symbols.cpp:342:118: error: invalid conversion from 'unsigned char*' to 'char*' [-fpermissive]
  342 | DEFINEFUNC7(int, PEM_write_bio_PKCS8PrivateKey, BIO *a, a, EVP_PKEY *b, b, const EVP_CIPHER *c, c, unsigned char *d, d, int e, e, pem_password_cb *f, f, void *g, g, return 0, return)
      |                                                                                                                      ^
      |                                                                                                                      |
      |                                                                                                                      unsigned char*
x509/openssl_symbols_p.h:237:84: note: in definition of macro 'DEFINEFUNC7'
  237 |     ret q_##func(arg1, arg2, arg3, arg4, arg5, arg6, arg7) { funcret func(a, b, c, d, e, f, g); }
      |                                                                                    ^
In file included from x509/openssl_symbols_p.h:64,
                 from x509/openssl_symbols.cpp:57:
/usr/include/openssl/pem.h:330:35: note:   initializing argument 4 of 'int PEM_write_bio_PKCS8PrivateKey(BIO*, EVP_PKEY*, const EVP_CIPHER*, char*, int, int (*)(char*, int, int, void*), void*)'
  330 |                                   char *, int, pem_password_cb *, void *);
      |                                   ^~~~~~
make[2]: *** [Makefile:2717: .obj/openssl_symbols.o] Error 1

Could you check that this PR is appropriate for fix that issue. Compilation with that fix finished without problems.