selftests/gpio: Add to CLEAN rule rather than overriding

Rather than overriding the CLEAN rule we can just append to it.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/gpio/Makefile

@@ -20,10 +20,7 @@ GPIODIR := $(realpath ../../../gpio)
 GPIOOUT := $(OUTPUT)/tools-gpio/
 GPIOOBJ := $(GPIOOUT)/gpio-utils.o
 
-override define CLEAN
-	$(RM) $(TEST_GEN_PROGS_EXTENDED)
-	$(RM) -rf $(GPIOOUT)
-endef
+CLEAN += ; $(RM) -rf $(GPIOOUT)
 
 $(TEST_GEN_PROGS_EXTENDED): $(GPIOOBJ)