MySQL: Fix qAddPostRoutine of mysql_server_end() on Windows

The function is declared as STDCALL, so we can't add a direct function
pointer to it (calling convention doesn't match what QtCore will try to
use). Instead, add a lambda as a trampoline.

Task-number: QTBUG-65471
Change-Id: I39332e0a867442d58082fffd1504002206e5cfaf
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/plugins/sqldrivers/mysql/qsql_mysql.cpp

@@ -1160,7 +1160,7 @@ static void qLibraryInit()
 #endif // Q_NO_MYSQL_EMBEDDED
 
 #ifdef MARIADB_BASE_VERSION
-    qAddPostRoutine(mysql_server_end);
+    qAddPostRoutine([]() { mysql_server_end(); });
 #endif
 }