timens: Add struct seq_file forward declaration

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

Fixes: 04a8682a ("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