stlink: export write_debug_reg

Change-Id: I3944911d4a71dba4af48470ceb3e4850784a0e7d
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/580
Tested-by: jenkins
Reviewed-by: Mathias Küster <kesmtp@freenet.de>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

src/jtag/drivers/stlink_usb.c

@@ -1272,4 +1272,6 @@ struct stlink_layout_api_s stlink_usb_layout_api = {
 	.read_mem32 = stlink_usb_read_mem32,
 	/** */
 	.write_mem32 = stlink_usb_write_mem32,
+	/** */
+	.write_debug_reg = stlink_usb_write_debug_reg
 };