kconfig: clean generated *conf-cfg files

I accidentally dropped '*' in the previous renaming patch.

Revive it so that 'make mrproper' can clean the generated files.

Fixes: d86271af ("kconfig: rename generated .*conf-cfg to *conf-cfg")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/Makefile

@@ -206,4 +206,4 @@ filechk_conf_cfg = $(CONFIG_SHELL) $<
 $(obj)/%conf-cfg: $(src)/%conf-cfg.sh FORCE
 	$(call filechk,conf_cfg)
 
-clean-files += conf-cfg
+clean-files += *conf-cfg