minor GCC3 compile fix

(there's still 2 more, but there's burried into several levels of macros, so it's hard to narrow them down)

Originally committed as revision 9265 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/ppc/h264_altivec.c

@@ -717,7 +717,7 @@ static inline vector unsigned char diff_lt_altivec ( register vector unsigned ch
     register vector unsigned char diff = vec_subs(x, y);
     register vector unsigned char diffneg = vec_subs(y, x);
     register vector unsigned char o = vec_or(diff, diffneg); /* |x-y| */
-    o = vec_cmplt(o, a);
+    o = (vector unsigned char)vec_cmplt(o, a);
     return o;
 }