selftests: mqueue: simplify the Makefile

Use make's implict rule for building simple C programs.
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/mqueue/Makefile

 CFLAGS += -O2
+LDLIBS = -lrt -lpthread -lpopt
+TEST_PROGS := mq_open_tests mq_perf_tests
 
-all:
-	$(CC) $(CFLAGS) mq_open_tests.c -o mq_open_tests -lrt
-	$(CC) $(CFLAGS) -o mq_perf_tests mq_perf_tests.c -lrt -lpthread -lpopt
+all: $(TEST_PROGS)
 
 include ../lib.mk
 
@@ -11,8 +11,6 @@ override define RUN_TESTS
 	@./mq_perf_tests || echo "selftests: mq_perf_tests [FAIL]"
 endef
 
-TEST_PROGS := mq_open_tests mq_perf_tests
-
 override define EMIT_TESTS
 	echo "./mq_open_tests /test1 || echo \"selftests: mq_open_tests [FAIL]\""
 	echo "./mq_perf_tests || echo \"selftests: mq_perf_tests [FAIL]\""