drivers: target: Move prototype declaration of function to header file target_core_pr.h

Move prototype declaration of function
spc_parse_naa_6h_vendor_specific() from target_core_xcopy.c to header
file target_core_pr.h because it is used by more than one file.

This eliminates the following warning in target_core_spc.c:
drivers/target/target_core_spc.c:138:6: warning: no previous prototype for ‘spc_parse_naa_6h_vendor_specific’ [-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>