tools: testing: selftests: mq_perf_tests: Fix infinite loop on ARM

We can't use a char type to check for a negative return value since char
isn't guaranteed to be signed. Indeed, the char type tends to be unsigned on
ARM.
Signed-off-by: Ndann frazier <dann.frazier@canonical.com>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/mqueue/mq_perf_tests.c

@@ -536,10 +536,9 @@ int main(int argc, char *argv[])
 {
 	struct mq_attr attr;
 	char *option, *next_option;
-	int i, cpu;
+	int i, cpu, rc;
 	struct sigaction sa;
 	poptContext popt_context;
-	char rc;
 	void *retval;
 
 	main_thread = pthread_self();