Makefile: add kselftest-clean to PHONY target list

kselftest-clean isn't in the PHONY target list. Add it.
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1184,6 +1184,7 @@ PHONY += kselftest
 kselftest:
 	$(Q)$(MAKE) -C tools/testing/selftests run_tests
 
+PHONY += kselftest-clean
 kselftest-clean:
 	$(Q)$(MAKE) -C tools/testing/selftests clean