kbuild: modpost on vmlinux regardless of CONFIG_MODULES

Based on patch from: Magnus Damm <magnus@valinux.co.jp>
This has the advantage that all section mismatch checks are run regardless
of modules being enabled or not.

When running modpost on vmlinux output:
MODPOST vmlinux

When running modpost on modules output count of modules like this:
MODPOST 5 modules

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Makefile b/Makefile
index b06b1ea..22451c2 100644
--- a/Makefile
+++ b/Makefile
@@ -736,6 +736,7 @@
 # vmlinux image - including updated kernel symbols
 vmlinux: $(vmlinux-lds) $(vmlinux-init) $(vmlinux-main) $(kallsyms.o) FORCE
 	$(call if_changed_rule,vmlinux__)
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost $@
 	$(Q)rm -f .old_version
 
 # The actual objects are generated when descending,