target/user: Add an #include directive

Since this driver uses kmap_atomic(), include the highmem header file.
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Andy Grover <agrover@redhat.com>

drivers/target/target_core_user.c

@@ -27,6 +27,7 @@
 #include <linux/uio_driver.h>
 #include <linux/stringify.h>
 #include <linux/bitops.h>
+#include <linux/highmem.h>
 #include <net/genetlink.h>
 #include <scsi/scsi_common.h>
 #include <scsi/scsi_proto.h>