Fix warnings from new version of clang tidy.

Change-Id: I2c85c8addd2bec83fa3796bb3ca8e87f5523a6fc
diff --git a/test/vmapi/primary_with_secondaries/mailbox.c b/test/vmapi/primary_with_secondaries/mailbox.c
index 7dd47fe..934f741 100644
--- a/test/vmapi/primary_with_secondaries/mailbox.c
+++ b/test/vmapi/primary_with_secondaries/mailbox.c
@@ -44,7 +44,8 @@
  */
 static void next_permutation(char *s, size_t len)
 {
-	size_t i, j;
+	size_t i;
+	size_t j;
 
 	for (i = len - 2; i < len; i--) {
 		const char t = s[i];