mp_image: include config.h directly

This is needed for HAVE_SSE4_INTRINSICS. config.h used to be included as
a transitive dependency of vf.h, but the include statement was removed
from vf.h in 8f2ccba71b.

Also silence an unused variable warning that was introduced in the same
commit.

video/filter/vf_d3d11vpp.c

@@ -463,7 +463,6 @@ static bool test_conversion(int in, int out)
 
 static int control(struct vf_instance *vf, int request, void* data)
 {
-    struct vf_priv_s *p = vf->priv;
     switch (request){
     case VFCTRL_SEEK_RESET:
         flush_frames(vf);

video/mp_image.c

@@ -30,6 +30,7 @@
 
 #include "mpv_talloc.h"
 
+#include "config.h"
 #include "common/common.h"
 #include "mp_image.h"
 #include "sws_utils.h"