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>

scripts/Makefile.build

@@ -294,7 +294,7 @@ quiet_cmd_cc_lst_c = MKLST   $@
 $(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      $@

scripts/Makefile.lib

@@ -78,7 +78,7 @@ header-test-y	+= $(filter-out $(header-test-), \
 		$(wildcard $(addprefix $(srctree)/$(src)/, \
 		$(header-test-pattern-y)))))
 
-extra-$(CONFIG_HEADER_TEST) += $(addsuffix .s, $(header-test-y))
+extra-$(CONFIG_HEADER_TEST) += $(addsuffix .s, $(header-test-y) $(header-test-m))
 
 # Add subdir path