selftests: locking: add Makefile for locking test

Add Makefile for locking test.
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/locking/Makefile

+# SPDX-License-Identifier: GPL-2.0
+#
+# Makefile for locking/ww_mutx selftests
+
+# No binaries, but make sure arg-less "make" doesn't trigger "run_tests"
+all:
+
+TEST_PROGS := ww_mutex.sh
+
+include ../lib.mk