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>