Ensure use_ssl is set when using --disable-ssl.

Fixes linker failure:

```
Undefined symbols for architecture x86_64:
  "_use_ssl", referenced from:
      _process_arguments in nrpe-4fa9f6.o
      _handle_connection in nrpe-4fa9f6.o
      _read_packet in nrpe-4fa9f6.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

src/check_nrpe.c

@@ -77,7 +77,10 @@ extern char *log_file;
 
 #ifdef HAVE_SSL
 SSL *ssl;
+int use_ssl = TRUE;
 unsigned long ssl_opts = SSL_OP_ALL;
+#else
+int use_ssl = FALSE;
 #endif
 int have_log_opts = FALSE;
 SslParms sslprm = {

src/nrpe.c

@@ -61,6 +61,12 @@ int       rfc931_timeout=15;
 # endif
 #endif
 
+#ifdef HAVE_SSL
+int       use_ssl = TRUE;
+#else
+int       use_ssl = FALSE;
+#endif
+
 
 #define DEFAULT_COMMAND_TIMEOUT			60	/* default timeout for execution of plugins */
 #define MAXFD							64