Kbuild: enable -Wunused-macros warning for "make W=2"

We have lots of dead defines and macros in drivers, lets offer users a way
to detect and eventually remove them.
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.extrawarn

@@ -37,6 +37,7 @@ warning-2 += $(call cc-option, -Wlogical-op)
 warning-2 += $(call cc-option, -Wmissing-field-initializers)
 warning-2 += $(call cc-option, -Wsign-compare)
 warning-2 += $(call cc-option, -Wmaybe-uninitialized)
+warning-2 += $(call cc-option, -Wunused-macros)
 
 warning-3 := -Wbad-function-cast
 warning-3 += -Wcast-qual