selftests: futex: override clean in lib.mk to fix warnings

Add override for lib.mk clean to fix the following warnings from clean
target run.

Makefile:36: warning: overriding recipe for target 'clean'
../lib.mk:55: warning: ignoring old recipe for target 'clean'
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/futex/Makefile

@@ -32,9 +32,10 @@ override define EMIT_TESTS
 	echo "./run.sh"
 endef
 
-clean:
+override define CLEAN
 	for DIR in $(SUBDIRS); do		\
 		BUILD_TARGET=$(OUTPUT)/$$DIR;	\
 		mkdir $$BUILD_TARGET  -p;	\
 		make OUTPUT=$$BUILD_TARGET -C $$DIR $@;\
 	done
+endef