ad_liba52: Avoid using the swab function

Some OSs do not have it and OS/2 kLIBC does not activate it
without _BSD_SOURCE defined (which should not be required).

Reported-by: KO Myung-Hun
Reviewed-by: Reimar


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@36900 b3059339-0415-0410-9bf9-f77b7e298cf2