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

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

Makefile:24: 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/sync/Makefile

@@ -20,5 +20,4 @@ TESTS += sync_stress_merge.o
 
 sync_test: $(OBJS) $(TESTS)
 
-clean:
-	$(RM) sync_test $(OBJS) $(TESTS)
+EXTRA_CLEAN := sync_test $(OBJS) $(TESTS)