selftests: mqueue: allow extra cflags

Change from = to += in order to allows the user to pass whatever
CFLAGS they wish(E.g. pass the proper headers and librareis
(popt.h and libpopt.so) in cross-compiling)
Suggested-by: NMichael Ellerman <mpe@ellerman.id.au>
Signed-off-by: NBamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Acked-by: NMichael Ellerman <mpe@ellerman.id.au>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/mqueue/Makefile

-CFLAGS = -O2
+CFLAGS += -O2
 
 all:
 	$(CC) $(CFLAGS) mq_open_tests.c -o mq_open_tests -lrt