drivers: block: Mark function seq_printf_with_thousands_grouping() as static in drbd_proc.c

Mark function seq_printf_with_thousands_grouping() as static in
drbd/drbd_proc.c because it is not used outside this file.

This eliminates the following warning in drbd/drbd_proc.c:
drivers/block/drbd/drbd_proc.c:49:6: warning: no previous prototype for ‘seq_printf_with_thousands_grouping’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>