Don't define ssl_verify_callback_common when using --disable-ssl.

Fixes build failure:
```
In file included from ./nrpe.c:44:
../include/nrpe-ssl.h:47:50: error: unknown type name 'X509_STORE_CTX'
int ssl_verify_callback_common(int preverify_ok, X509_STORE_CTX * ctx, int is_invalid);
                                                 ^
```

include/nrpe-ssl.h

@@ -44,4 +44,6 @@ void ssl_set_protocol_version(SslVer ssl_proto_ver, unsigned long *ssl_opts);
 void ssl_log_startup(int server);
 int ssl_load_certificates(void);
 int ssl_set_ciphers(void);
+#ifdef HAVE_SSL
 int ssl_verify_callback_common(int preverify_ok, X509_STORE_CTX * ctx, int is_invalid);
+#endif