Unify the many definitions of NELEMENTS

Change-Id: Ie52ce24ca9eca272dfacfb17483ec7b7f2c126d4
diff --git a/aom/aom_integer.h b/aom/aom_integer.h
index 2e8f23f..bd1fe27 100644
--- a/aom/aom_integer.h
+++ b/aom/aom_integer.h
@@ -61,4 +61,6 @@
 #include <inttypes.h>
 #endif
 
+#define NELEMENTS(x) (int)(sizeof(x) / sizeof(x[0]))
+
 #endif  // AOM_AOM_INTEGER_H_