bswap.h was included since 7b114c09, yet since 3788a3c0 no explicit use
of anything from bswap.h has been made, so remove this header.

(Only AV_RL32 is used and while this might imply swapping on
big-endian systems, it is contained in libavutil/intreadwrite.h.)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>