modpost: turn missing MODULE_LICENSE() into error

Do not create modules with no license tag.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/mod/modpost.c

@@ -2018,7 +2018,7 @@ static void read_symbols(const char *modname)
 	if (!mod->is_vmlinux) {
 		license = get_modinfo(&info, "license");
 		if (!license)
-			warn("missing MODULE_LICENSE() in %s\n", modname);
+			error("missing MODULE_LICENSE() in %s\n", modname);
 		while (license) {
 			if (license_is_gpl_compatible(license))
 				mod->gpl_compatible = 1;