Define AOM_USAGE_* macros as unsigned constants

The AOM_USAGE_* macros are used with the "unsigned int g_usage" member
of the aom_codec_enc_cfg_t struct of the "unsigned int usage" parameter
of the aom_codec_enc_config_default() function. Defining these macros as
unsigned int constants allow them to be used in C++ templates with the
correct type inferred.

Change-Id: I31b423185948c6c5d3af789317af7beace27d34f

aom/aom_encoder.h

diff --git a/aom/aom_encoder.h b/aom/aom_encoder.h
index 5482c66..5d0bbe1 100644
--- a/aom/aom_encoder.h
+++ b/aom/aom_encoder.h
@@ -1006,11 +1006,11 @@
 aom_fixed_buf_t *aom_codec_get_global_headers(aom_codec_ctx_t *ctx);
 
 /*!\brief usage parameter analogous to AV1 GOOD QUALITY mode. */
-#define AOM_USAGE_GOOD_QUALITY 0
+#define AOM_USAGE_GOOD_QUALITY 0u
 /*!\brief usage parameter analogous to AV1 REALTIME mode. */
-#define AOM_USAGE_REALTIME 1
+#define AOM_USAGE_REALTIME 1u
 /*!\brief usage parameter analogous to AV1 all intra mode. */
-#define AOM_USAGE_ALL_INTRA 2
+#define AOM_USAGE_ALL_INTRA 2u
 
 /*!\brief Encode a frame
  *