kbuild: when warning symbols exported twice now tell user this is the problem

Warning now looks like this:
WARNING: vmlinux: 'strcpy' exported twice. Previous export was in vmlinux

Which gives much better hint how to fix it.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 3648683..e2bf4c9 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -191,7 +191,7 @@
 		s = new_symbol(name, mod);
 	} else {
 		if (!s->preloaded) {
-			warn("%s: duplicate symbol '%s' previous definition "
+			warn("%s: '%s' exported twice. Previous export "
 			     "was in %s%s\n", mod->name, name,
 			     s->module->name,
 			     is_vmlinux(s->module->name) ?"":".ko");