selftests: gpio: fix clean target to remove all generated files and dirs

gpio test creates executables, object files, and include directory
under selftests directory. Enhance clean target to remove all files
it generates.

Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/gpio/Makefile

···
 TEST_PROGS := gpio-mockup.sh
 TEST_FILES := gpio-mockup-sysfs.sh $(BINARIES)
 BINARIES := gpio-mockup-chardev
+EXTRA_PROGS := ../gpiogpio-event-mon ../gpiogpio-hammer ../gpiolsgpio
+EXTRA_DIRS := ../gpioinclude/
+EXTRA_OBJS := ../gpiogpio-event-mon-in.o ../gpiogpio-event-mon.o
+EXTRA_OBJS += ../gpiogpio-hammer-in.o ../gpiogpio-utils.o ../gpiolsgpio-in.o
+EXTRA_OBJS += ../gpiolsgpio.o
 
 include ../lib.mk
 
 all: $(BINARIES)
 
 override define CLEAN
-	$(RM) $(BINARIES)
+	$(RM) $(BINARIES) $(EXTRA_PROGS) $(EXTRA_OBJS)
+	$(RM) -r $(EXTRA_DIRS)
 endef
 
 CFLAGS += -O2 -g -std=gnu99 -Wall -I../../../../usr/include/