fs: fix missing declaration of init_files

fs/file.c should include include/linux/init_task.h  for
 declaration of init_files. This fixes the sparse warning:

fs/file.c:501:21: warning: symbol 'init_files' was not declared. Should it be static?

Signed-off-by: Zhang Kunbo <zhangkunbo@huawei.com>
Link: https://lore.kernel.org/r/20241217071836.2634868-1-zhangkunbo@huawei.com


Signed-off-by: Christian Brauner <brauner@kernel.org>