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>

drivers/target/iscsi/iscsi_target.c

@@ -3468,7 +3468,12 @@ static inline void iscsit_thread_check_cpumask(
 }
 
 #else
-#define iscsit_thread_get_cpumask(X) ({})
+
+void iscsit_thread_get_cpumask(struct iscsi_conn *conn)
+{
+	return;
+}
+
 #define iscsit_thread_check_cpumask(X, Y, Z) ({})
 #endif /* CONFIG_SMP */