Fix fwd kf overlay frame's type

The overlay frame associated with a forward key frame is set to
be an inter frame.

BUG=aomedia:3079

Change-Id: Ief59cccc9b975de22af6396a16e0341a9c049e2c
diff --git a/av1/encoder/gop_structure.c b/av1/encoder/gop_structure.c
index 1c06545..45c5755 100644
--- a/av1/encoder/gop_structure.c
+++ b/av1/encoder/gop_structure.c
@@ -725,7 +725,7 @@
     gf_group->cur_frame_idx[frame_index] = cur_frame_index;
     gf_group->layer_depth[frame_index] = MAX_ARF_LAYERS;
     gf_group->arf_boost[frame_index] = NORMAL_BOOST;
-    gf_group->frame_type[frame_index] = is_fwd_kf ? KEY_FRAME : INTER_FRAME;
+    gf_group->frame_type[frame_index] = INTER_FRAME;
     gf_group->refbuf_state[frame_index] =
         is_fwd_kf ? REFBUF_RESET : REFBUF_UPDATE;
 #if CONFIG_FRAME_PARALLEL_ENCODE