tests/date.c: include config.h to expose Vista+ macros

date.c uses SUBLANG_LITHUANIAN_LITHUANIA which is Vista+
Include config.h so that _WIN32_WINNT is defined and the newer macros
are exposed.

This fixes the build under MinGW.

https://bugzilla.gnome.org/show_bug.cgi?id=793729

glib/tests/date.c

@@ -4,6 +4,8 @@
 /* We are testing some deprecated APIs here */
 #define GLIB_DISABLE_DEPRECATION_WARNINGS
 
+#include "config.h"
+
 #include "glib.h"
 
 #include <stdio.h>