yadif: Re-renable MMX asm on x86_64 with meson

The meson build doesn't automatically set HAVE_CPU_* defines
like autotools did, so the yadif plugin was being built without
the MMX assembler support

gst/yadif/meson.build

@@ -4,9 +4,17 @@ yadif_sources = [
   'yadif.c'
 ]
 
+local_c_args = gst_plugins_bad_args
+
+# The plugin can build asm extensions
+# on x86_64
+if host_machine.cpu() == 'x86_64'
+  local_c_args += ['-DHAVE_CPU_X86_64=1']
+endif
+
 gstyadif = library('gstyadif',
   yadif_sources,
-  c_args : gst_plugins_bad_args,
+  c_args : local_c_args,
   include_directories : [configinc],
   dependencies : [gstbase_dep, gstvideo_dep],
   install : true,