Fix build: use proper gcc attribute syntax

Change-Id: I4c5d962bcac389d86eadb4bccc838a017bda4062

av1/common/onyxc_int.h

diff --git a/av1/common/onyxc_int.h b/av1/common/onyxc_int.h
index 3f4c4d3..0aff79c 100644
--- a/av1/common/onyxc_int.h
+++ b/av1/common/onyxc_int.h
@@ -48,7 +48,7 @@
 #define AOM_FALLTHROUGH_INTENDED [[clang::fallthrough]]  // NOLINT
 #endif
 #elif defined(__GNUC__) && __GNUC__ >= 7
-#define AOM_FALLTHROUGH_INTENDED [[gnu::fallthrough]]  // NOLINT
+#define AOM_FALLTHROUGH_INTENDED __attribute__((fallthrough))  // NOLINT
 #endif
 
 #ifndef AOM_FALLTHROUGH_INTENDED