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 <[e-mail]>

tools/testing/selftests/splice/Makefile

···
 
 include ../lib.mk
 
-clean:
-	rm -fr $(EXTRA)
+EXTRA_CLEAN := $(EXTRA)