Only provide openssl_tls_init_hook if building with openssl

This should have been protected by #ifdef USE_OPENSSL in commit
896fcdb230.

Per the real complaint this time from Daniel Gustafsson.

src/include/libpq/libpq-be.h

@@ -288,8 +288,10 @@ extern char *be_tls_get_certificate_hash(Port *port, size_t *len);
 #endif
 
 /* init hook for SSL, the default sets the password callback if appropriate */
+#ifdef USE_OPENSSL
 typedef void(* openssl_tls_init_hook_typ)(SSL_CTX *context, bool isServerStart);
 extern PGDLLIMPORT openssl_tls_init_hook_typ openssl_tls_init_hook;
+#endif
 
 #endif							/* USE_SSL */