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

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

Makefile:63: warning: overriding recipe for target 'clean'
../lib.mk:55: warning: ignoring old recipe for target 'clean'
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/powerpc/Makefile

@@ -59,12 +59,13 @@ override define EMIT_TESTS
 	done;
 endef
 
-clean:
+override define CLEAN
 	@for TARGET in $(SUB_DIRS); do \
 		BUILD_TARGET=$$OUTPUT/$$TARGET;	\
 		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET clean; \
 	done;
 	rm -f tags
+endef
 
 tags:
 	find . -name '*.c' -o -name '*.h' | xargs ctags