selftests/futex: fix build for clang

stable inclusion
from stable-v5.10.156
commit fdf6807606293aa148254510fa14d8f88e2a8984
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7MCG1

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fdf6807606293aa148254510fa14d8f88e2a8984



--------------------------------

[ Upstream commit 03cab65a ]

Don't use the test-specific header files as source files to force a
target dependency, as clang will complain if more than one source file
is used for a compile command with a single '-o' flag.

Use the proper Makefile variables instead as defined in
tools/testing/selftests/lib.mk.

Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>