Declare struct SwsContext before using it, fixes the checkheaders warning:
libswscale/swscale_internal.h:58: warning: `struct SwsContext' declared inside parameter list
libswscale/swscale_internal.h:58: warning: its scope is only this definition or declaration, which is probably not what you want

Originally committed as revision 28353 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale
