selftests/futex: fix build for clang

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>