playlist demux: do not use index()

marked as LEGACY in POSIX.1-2001.
POSIX.1-2008 removes the  specifications  of  index()
and rindex(), recommending strchr(3) and strrchr(3) instead.

modules/demux/playlist/playlist.c

@@ -241,7 +241,7 @@ bool CheckContentType( stream_t * p_stream, const char * psz_ctype )
     }
 
     /* check for Content-Type: foo-type; charset=... */
-    const char * psz_sep = index( psz_check, ';' );
+    const char * psz_sep = strchr( psz_check, ';' );
     if ( psz_sep )
         i_len = __MIN( i_len, psz_sep - psz_check );