kbuild: show warning if 'make headers_check' is used

Since commit 7ecaf069 ("kbuild: move headers_check rule to
usr/include/Makefile"), 'make headers_check' is no-op.

This stub target is remaining here in case some scripts still invoke it.
In order to prompt people to remove stale code, show a noisy warning
message if used. The stub will be really removed after the Linux 5.15
release.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -1358,7 +1358,11 @@ headers: $(version_h) scripts_unifdef uapi-asm-generic archheaders archscripts
 # Deprecated. It is no-op now.
 PHONY += headers_check
 headers_check:
-	@:
+	@echo >&2 "=================== WARNING ==================="
+	@echo >&2 "Since Linux 5.5, 'make headers_check' is no-op,"
+	@echo >&2 "and will be removed after Linux 5.15 release."
+	@echo >&2 "Please remove headers_check from your scripts."
+	@echo >&2 "==============================================="
 
 ifdef CONFIG_HEADERS_INSTALL
 prepare: headers