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

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

Makefile:11: 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/gpio/Makefile

@@ -7,8 +7,9 @@ include ../lib.mk
 
 all: $(BINARIES)
 
-clean:
+override define CLEAN
 	$(RM) $(BINARIES)
+endef
 
 CFLAGS += -O2 -g -std=gnu99 -Wall -I../../../../usr/include/
 LDLIBS += -lmount -I/usr/include/libmount