swscale: Readd #define _SVID_SOURCE

This was removed erroneously in
046f081b46c8479820409cf8f530b988221bd15b. This define still is
necessary for getting MAP_ANONYMOUS defined on linux/glibc,
despite the define reshuffling done in that commit.

Without MAP_ANONYMOUS defined, the mprotect calls for setting the
generated mmx2 scaler code pages executable are left out, causing
crashes if that codepath is chosen.

This patch fixes scaling from 192x144 to 320x240 with
-sws_flags fast_bilinear, which crashes on linux at the
moment.

Signed-off-by: Martin Storsjö <martin@martin.st>
