gtk2: workaround knownfolders.h linker issue

As of mingw-w64 r5589 the header knownfolders.h was added which contains
definitions like FOLDERID_Windows As these are also part of libuuid.a
you'll get linker failures like:

/usr/i686-w64-mingw32/sys-root/mingw/lib/libuuid.a(lib32_libuuid_a-uuid.o):(.rdata+0x0):multiple definition of `FOLDERID_Windows'
win32/.libs/libgdk-win32.a(gdkdnd-win32.o):gdkdnd-win32.c:(.rdata+0x360):first defined here

Workaround this for now until mingw-w64 upstream has come up with a
statement whether this is expected behaviour and what component should
be fixed exactly:

http://mingw-w64.svn.sourceforge.net/viewvc/mingw-w64?view=revision&revision=5589