selftests: splice: 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:8: 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/splice/Makefile

@@ -4,5 +4,4 @@ all: $(TEST_PROGS) $(EXTRA)
 
 include ../lib.mk
 
-clean:
-	rm -fr $(EXTRA)
+EXTRA_CLEAN := $(EXTRA)