reset: hi6220: fix modular build

We need to include <linux/module.h> to build the driver as a loadable
module:

drivers/reset/hisilicon/hi6220_reset.c:108:1: warning: data definition has no type or storage class
 postcore_initcall(hi6220_reset_init);

Signed-off-by: Arnd Bergmann <arnd@arndb.de>