gtk: avoid redefining _WIN32_WINNT macro

gtk: avoid redefining _WIN32_WINNT macro

When building for Mingw64 target on Fedora 22 a warning
is issued about _WIN32_WINNT being redefined.

In file included from ui/gtk.c:40:0:
include/ui/gtk.h:5:0: warning: "_WIN32_WINNT" redefined
 # define _WIN32_WINNT 0x0601 /* needed to get definition of MAPVK_VK_TO_VSC */
  ^
In file included from /usr/i686-w64-mingw32/sys-root/mingw/include/crtdefs.h:10:0,
                 from /usr/i686-w64-mingw32/sys-root/mingw/include/stdio.h:9,
                 from /home/berrange/src/virt/qemu/include/qemu/fprintf-fn.h:12,
                 from /home/berrange/src/virt/qemu/include/qemu-common.h:18,
                 from ui/gtk.c:37:
/usr/i686-w64-mingw32/sys-root/mingw/include/_mingw.h:225:0: note: this is the location of the previous definition
 #define _WIN32_WINNT 0x502
 ^

Rather than try to get MAPVK_VK_TO_VSC defined indirectly
by defining _WIN32_WINNT, instead just define it explicitly
if missing.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>