db_mysql: fix build for older releases

> km_my_con.c: In function 'db_mysql_new_connection':
> km_my_con.c:132:4: warning: implicit declaration of function 'mysql_optionsv' [-Wimplicit-function-declaration]
>      mysql_optionsv(ptr->con, MYSQL_OPT_SSL_ENFORCE, (void *)&(int){1});
>      ^~~~~~~~~~~~~~
> km_my_con.c:132:29: error: 'MYSQL_OPT_SSL_ENFORCE' undeclared (first use in this function)
>     mysql_optionsv(ptr->con, MYSQL_OPT_SSL_ENFORCE, (void *)&(int){1});
>                              ^~~~~~~~~~~~~~~~~~~~~
> km_my_con.c:132:29: note: each undeclared identifier is reported only once for each function it appears in
> ../../Makefile.rules:100: recipe for target 'km_my_con.o' failed

src/modules/db_mysql/km_my_con.c

@@ -117,6 +117,7 @@ struct my_con *db_mysql_new_connection(const struct db_id *id)
 	mysql_options(ptr->con, MYSQL_OPT_WRITE_TIMEOUT,
 			(const void *)&db_mysql_timeout_interval);
 
+#if MYSQL_VERSION_ID > 50710
 #ifdef MARIADB_BASE_VERSION
 	/*
 	 * emulate SSL_MODE_XXXX from MySQL
@@ -141,7 +142,6 @@ struct my_con *db_mysql_new_connection(const struct db_id *id)
 			break;
 	}
 #else
-#if MYSQL_VERSION_ID > 50710
 	if(db_mysql_opt_ssl_mode != 0) {
 		unsigned int optuint = 0;
 		if(db_mysql_opt_ssl_mode == 1) {
@@ -155,14 +155,14 @@ struct my_con *db_mysql_new_connection(const struct db_id *id)
 		}
 		mysql_options(ptr->con, MYSQL_OPT_SSL_MODE, (const void *)&optuint);
 	}
+#endif /* MARIADB_BASE_VERSION */
 #else
 	if(db_mysql_opt_ssl_mode != 0) {
 		LM_WARN("ssl mode not supported by mysql version (value %u) - "
 				"ignoring\n",
 				(unsigned int)db_mysql_opt_ssl_mode);
 	}
 #endif /* MYSQL_VERSION_ID */
-#endif /* MARIADB_BASE_VERSION */
 
 #if MYSQL_VERSION_ID > 50012
 	/* set reconnect flag if enabled */