Rename NONE to NONE_FRAME.

This follows the naming for the other frame types, and allows libaom
to be compiled against other libraries that also #define NONE.

Change-Id: Ic2e2814587bbc5ea67385a9af775396d29b7dde0
diff --git a/av1/common/enums.h b/av1/common/enums.h
index 530e304..5fe3b2d 100644
--- a/av1/common/enums.h
+++ b/av1/common/enums.h
@@ -458,7 +458,7 @@
 typedef uint8_t TXFM_CONTEXT;
 #endif
 
-#define NONE -1
+#define NONE_FRAME -1
 #define INTRA_FRAME 0
 #define LAST_FRAME 1