remove definition of swab

right now swab is defined in libavfilter/libmpcodecs/mpbswap.h
this breaks mingw32 cross compiles. since it its not used, it should be
removed. alternatively swab has to be added to HAVE_LIST in configure.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/libmpcodecs/mpbswap.h

@@ -31,8 +31,4 @@
 #define be2me_16(v) av_be2ne16(v)
 #define be2me_32(v) av_be2ne32(v)
 
-#ifndef HAVE_SWAB
-void swab(const void *from, void *to, ssize_t n);
-#endif
-
 #endif /* MPLAYER_MPBSWAP_H */