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: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>