timens: Add struct seq_file forward declaration

Add forward declaration of struct seq_file before using it in a
function prototype.

Fixes: 04a8682 ("fs/proc: Introduce /proc/pid/timens_offsets")
Signed-off-by: Herbert Xu <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Andrei Vagin <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

Author: herbertx

include/linux/time_namespace.h

@@ -12,6 +12,7 @@
 struct user_namespace;
 extern struct user_namespace init_user_ns;
 
+struct seq_file;
 struct vm_area_struct;
 
 struct timens_offsets {