avformat/os_support: Add _DEFAULT_SOURCE to hide warning about _SVID_SOURCE depreciation

As suggested, this just adds _DEFAULT_SOURCE while preserving
_SVID_SOURCE for whatever old systems still need it.
Signed-off-by: Daniel Bomar <dbdaniel42@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/os_support.c

@@ -21,6 +21,7 @@
  */
 
 /* needed by inet_aton() */
+#define _DEFAULT_SOURCE
 #define _SVID_SOURCE
 
 #include "config.h"