drivers: target: Mark functions as static in tcm_loop.c

Mark functions tcm_loop_make_naa_tpg(), tcm_loop_drop_naa_tpg(),
tcm_loop_make_scsi_hba() and tcm_loop_drop_scsi_hba() as static in
loopback/tcm_loop.c because they are not used outside this file.

This eliminates the following warning in loopback/tcm_loop.c:
drivers/target/loopback/tcm_loop.c:1231:25: warning: no previous prototype for ‘tcm_loop_make_naa_tpg’ [-Wmissing-prototypes]
drivers/target/loopback/tcm_loop.c:1276:6: warning: no previous prototype for ‘tcm_loop_drop_naa_tpg’ [-Wmissing-prototypes]
drivers/target/loopback/tcm_loop.c:1308:16: warning: no previous prototype for ‘tcm_loop_make_scsi_hba’ [-Wmissing-prototypes]
drivers/target/loopback/tcm_loop.c:1378:6: warning: no previous prototype for ‘tcm_loop_drop_scsi_hba’ [-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>