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 <herbert@gondor.apana.org.au>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andrei Vagin <avagin@gmail.com>
Link: https://lore.kernel.org/all/aDlskzKIAULMlwPj@gondor.apana.org.au

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 {