aom: fix compilation with older packages

AOM_USAGE_ALL_INTRA isn't being defined while AOM_USAGE_GOOD_QUALITY is, refs
https://aomedia.googlesource.com/aom/+/refs/tags/v3.0.0/aom/aom_encoder.h#1001

modules/codec/aom.c

@@ -46,7 +46,11 @@
 
 #ifndef AOM_USAGE_GOOD_QUALITY
 # define AOM_USAGE_GOOD_QUALITY 0
+#endif
+#ifndef AOM_USAGE_REALTIME
 # define AOM_USAGE_REALTIME 1
+#endif
+#ifndef AOM_USAGE_ALL_INTRA
 # define AOM_USAGE_ALL_INTRA 2
 #endif