eliminate some mingw warnings

Signed-off-by: rogerdpack <rogerpack2005@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/os_support.h

@@ -37,7 +37,13 @@
 #   undef lseek
 #  endif
 #  define lseek(f,p,w) _lseeki64((f), (p), (w))
+#  ifdef stat
+#   undef stat
+#  endif
 #  define stat _stati64
+#  ifdef fstat
+#   undef fstat
+#  endif
 #  define fstat(f,s) _fstati64((f), (s))
 #endif /* defined(__MINGW32__) && !defined(__MINGW32CE__) */