fs: add <linux/init_task.h> for 'init_fs'

The init_fs symbol is defined in <linux/init_task.h> but was
not included in fs/fs_struct.c so fix by adding the include.

Fixes the following sparse warning:
fs/fs_struct.c:150:18: warning: symbol 'init_fs' was not declared. Should it be static?

Fixes: 3e93cd6 ("Take fs_struct handling to new file")
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://patch.msgid.link/20260108115856.238027-1-ben.dooks@codethink.co.uk
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: bjdooks-ct

fs/fs_struct.c

@@ -6,6 +6,7 @@
 #include <linux/path.h>
 #include <linux/slab.h>
 #include <linux/fs_struct.h>
+#include <linux/init_task.h>
 #include "internal.h"
 
 /*