aom_codec.h: make flag constants unsigned

this matches the type of aom_codec_frame_flags_t; a similar change was
made in libvpx:

9db0ec67e vpx_encoder.h: make flag constants unsigned

Change-Id: I34a1a1873a7c7c046e2748250ed600e672b96bea
diff --git a/av1/encoder/thirdpass.c b/av1/encoder/thirdpass.c
index a97efe9..a25522f 100644
--- a/av1/encoder/thirdpass.c
+++ b/av1/encoder/thirdpass.c
@@ -127,7 +127,7 @@
     aom_internal_error(ctx->err_info, AOM_CODEC_ERROR,
                        "Third pass frame info ran out of available slots.");
   }
-  int frame_type_flags = 0;
+  aom_codec_frame_flags_t frame_type_flags = 0;
   if (aom_codec_control(&ctx->decoder, AOMD_GET_FRAME_FLAGS,
                         &frame_type_flags) != AOM_CODEC_OK) {
     aom_internal_error(ctx->err_info, AOM_CODEC_ERROR,