dsputil: Use explicit movzbl instead of movzx

This fixes compilation with the latest clang trunk version.

Patch by İsmail Dönmez, ismail at namtrac dot org

Originally committed as revision 25628 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/x86/dsputil_mmx.c

@@ -612,7 +612,7 @@ static void add_hfyu_median_prediction_cmov(uint8_t *dst, const uint8_t *top, co
     __asm__ volatile(
         "mov    %7, %3 \n"
         "1: \n"
-        "movzx (%3,%4), %2 \n"
+        "movzbl (%3,%4), %2 \n"
         "mov    %2, %k3 \n"
         "sub   %b1, %b3 \n"
         "add   %b0, %b3 \n"