avfilter/vf_gblur: add missing arch check

Removed by mistake in 2b4da1cb8c where it
should have been replaced instead.

Signed-off-by: James Almer <jamrial@gmail.com>

libavfilter/vf_gblur.c

@@ -234,6 +234,7 @@ void ff_gblur_init(GBlurContext *s)
 {
     s->horiz_slice = horiz_slice_c;
     s->postscale_slice = postscale_c;
+    if (ARCH_X86)
         ff_gblur_init_x86(s);
 }