[build] Fix compiler warning on OpenBSD 5.7

Reported-by: Jiri B <jirib@devio.us>
Tested-by: Jiri B <jirib@devio.us>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/transitions/librm_mgmt.c

 				( uint32_t ) vec->next );
 		set_interrupt_vector ( intr, vec );
 	}
-	DBGC ( &intr_vec[0], "INTn vector at %p+%xn (phys %#lx+%xn)\n",
+	DBGC ( &intr_vec[0], "INTn vector at %p+%zxn (phys %#lx+%zxn)\n",
 	       intr_vec, sizeof ( intr_vec[0] ),
 	       virt_to_phys ( intr_vec ), sizeof ( intr_vec[0] ) );