Fix up iscsiboot missing prototype warnings

src/include/usr/iscsiboot.h

+#ifndef _USR_ISCSIBOOT_H
+#define _USR_ISCSIBOOT_H
+
+extern int iscsiboot ( const char *root_path );
+
+#endif /* _USR_ISCSIBOOT_H */

src/usr/autoboot.c

 #include <usr/route.h>
 #include <usr/dhcpmgmt.h>
 #include <usr/imgmgmt.h>
+#include <usr/iscsiboot.h>
 #include <usr/autoboot.h>
 
 /** @file

src/tests/iscsiboot.c → src/usr/iscsiboot.c

 #include <gpxe/iscsi.h>
 #include <gpxe/dhcp.h>
 #include <int13.h>
+#include <usr/iscsiboot.h>
 
 int iscsiboot ( const char *root_path ) {
 	struct scsi_device scsi;