os_support: Undefine lseek/stat/fstat before defining them

This avoids a number of redefinition warnings on MinGW64.

libavformat/os_support.h

@@ -33,8 +33,11 @@
 
 #if defined(_WIN32) && !defined(__MINGW32CE__)
 #  include <fcntl.h>
+#  undef lseek
 #  define lseek(f,p,w) _lseeki64((f), (p), (w))
+#  undef stat
 #  define stat _stati64
+#  undef fstat
 #  define fstat(f,s) _fstati64((f), (s))
 #endif /* defined(__MINGW32__) && !defined(__MINGW32CE__) */