fix(rmm-eac5): fix MISRA C 12.1 violations

Change-Id: I571a8d32efdb8caf4586c3580adccac0a0f81e22
Signed-off-by: AlexeiFedorov <Alexei.Fedorov@arm.com>
diff --git a/lib/common/include/utils_def.h b/lib/common/include/utils_def.h
index f028482..ba4f60e 100644
--- a/lib/common/include/utils_def.h
+++ b/lib/common/include/utils_def.h
@@ -223,7 +223,7 @@
 #define SET_MEMBER(member, start, end)	\
 	union {				\
 		member;			\
-		unsigned char reserved##end[(end) - (start)]; \
+		unsigned char reserved##end[((end) - (start))]; \
 	}
 
 #define FALLTHROUGH	__attribute__((fallthrough))