videomixer: use 'q' constraint instead of 'r'

This avoids the "bad register name `%dil'" compilation errors on 32bit where
because of 'r' gcc puts the value in a general purpose register and then tries
to access the lower part as %dil/%sil which is not existing on 32bit. 'q' requests
a-d registers

gst/videomixer/blend_mmx.h

@@ -233,8 +233,8 @@ _memset_u8_mmx (guint8 * dest, guint val, guint count)
     "jne        4b            \n\t"
     "5:                       \n\t"
     "emms                     \n\t"
-    : "=r" (count), "=r" (dest)
-    : "0" (count), "1" (dest), "r" (val8), "m" (val64)
+    : "=r" (count), "=q" (dest)
+    : "0" (count), "1" (dest), "q" (val8), "m" (val64)
     : "memory"
 #ifdef __MMX__
       , "mm0"