selftests: cgroup: build error multiple outpt files

When building selftests/cgroup: with clang the following error are seen:

clang -Wall -pthread    test_memcontrol.c cgroup_util.c ../clone3/clone3_selftests.h  -o .../builds/current/kselftest/cgroup/test_memcontrol
clang: error: cannot specify -o when generating multiple output files
make[3]: *** [../lib.mk:146: .../builds/current/kselftest/cgroup/test_memcontrol] Error 1

Rework to add the header files to LOCAL_HDRS before including ../lib.mk,
since the dependency is evaluated in '$(OUTPUT)/%:%.c $(LOCAL_HDRS)' in
file lib.mk.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>