tests: Fix compile warning on mingw64

tests: Fix compile warning on mingw64

In file included from ../../libs/gst/net/gstntppacket.c:35:0,
                 from netclock-replay.c:25:
../../config.h:546:0: error: "__MSVCRT_VERSION__" redefined [-Werror]
 #define __MSVCRT_VERSION__ 0x0601

In file included from /usr/x86_64-w64-mingw32/sys-root/mingw/include/crtdefs.h:10:0,
                 from /usr/x86_64-w64-mingw32/sys-root/mingw/include/stdio.h:9,
                 from netclock-replay.c:21:
/usr/x86_64-w64-mingw32/sys-root/mingw/include/_mingw.h:220:0: note:
this is the location of the previous definition
 # define __MSVCRT_VERSION__ 0x0700

https://bugzilla.gnome.org/show_bug.cgi?id=774108