iscsi-target: Fix CONFIG_SMP=n and CONFIG_MODULES=n build failure

iscsi-target: Fix CONFIG_SMP=n and CONFIG_MODULES=n build failure

This patch fixes the following CONFIG_SMP=n and CONFIG_MODULES=n build
failure, because iscsit_thread_get_cpumask() is defined as a macro in
iscsi_target.c, but needed by iscsi_target_login.c

drivers/built-in.o: In function `iscsi_post_login_handler':
iscsi_target_login.c:(.text+0x13a315): undefined reference to `iscsit_thread_get_cpumask'
iscsi_target_login.c:(.text+0x13a4b4): undefined reference to `iscsit_thread_get_cpumask'
make: *** [.tmp_vmlinux1] Error 1

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>