drivers: target: Mark function as static in target_core_iblock.c

Mark function iblock_get_write_cache() as static in target_core_iblock.c
because it is not used outside this file.

This eliminates the following warning in target_core_iblock.c:
drivers/target/target_core_iblock.c:766:6: warning: no previous prototype for ‘iblock_get_write_cache’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>