Simplify conditional for gf_group->is_sframe_due

Change !(x == y) to x != y.

Change-Id: I5544948c008bf3d91ed0760781ce597f9777ecb0

av1/encoder/gop_structure.c

diff --git a/av1/encoder/gop_structure.c b/av1/encoder/gop_structure.c
index c239502..308290f 100644
--- a/av1/encoder/gop_structure.c
+++ b/av1/encoder/gop_structure.c
@@ -642,7 +642,7 @@
                                         : gf_group->is_sframe_due ? S_FRAME
                                                                   : INTER_FRAME;
     gf_group->is_sframe_due =
-        sframe_enabled && !(gf_group->frame_type[frame_index] == S_FRAME);
+        sframe_enabled && gf_group->frame_type[frame_index] != S_FRAME;
     gf_group->refbuf_state[frame_index] = REFBUF_UPDATE;
     gf_group->max_layer_depth = 1;
     gf_group->arf_index = frame_index;