MFT: GUID_AC3 should be defined in Mingw-w64 4.0

Of course, it's technically a GCC bug, because selectany is broken, but
there is no simple fix for that.

(cherry picked from commit 48d665494c2d23e474081a8b222942068b74b9b3)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/mft.c

@@ -172,7 +172,9 @@ static const pair_format_guid video_format_table[] =
     { 0, NULL }
 };
 
+#if defined(__MINGW64_VERSION_MAJOR) && __MINGW64_VERSION_MAJOR < 4
 DEFINE_GUID(MFAudioFormat_Dolby_AC3, 0xe06d802c, 0xdb46, 0x11cf, 0xb4, 0xd1, 0x00, 0x80, 0x5f, 0x6c, 0xbb, 0xea);
+#endif
 /*
  * We cannot use the FOURCC code for audio either since the
  * WAVE_FORMAT value is used to create the GUID.