kbuild: compile-test headers listed in header-test-m as well

It will be useful to control the header-test by a tristate option.

If CONFIG_FOO is a tristate option, you can write like this:

  header-test-$(CONFIG_FOO) += foo.h

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index be38198..5829ccb 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -294,7 +294,7 @@
 $(obj)/%.lst: $(src)/%.c FORCE
 	$(call if_changed_dep,cc_lst_c)
 
-# header test (header-test-y target)
+# header test (header-test-y, header-test-m target)
 # ---------------------------------------------------------------------------
 
 quiet_cmd_cc_s_h = CC      $@