DWT: x86 init should depend on HAVE_MMX

The init function is only compiled if MMX is enabled, the call
must use the same condition.

Originally committed as revision 22531 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/dwt.c

@@ -839,5 +839,5 @@ void ff_dwt_init(DWTContext *c)
     c->horizontal_compose97i = ff_snow_horizontal_compose97i;
     c->inner_add_yblock = ff_snow_inner_add_yblock;
 
-    if (ARCH_X86) ff_dwt_init_x86(c);
+    if (HAVE_MMX) ff_dwt_init_x86(c);
 }