fix compile error

Reference: /n/atom/patch/applied2013/bitsyuart
Date: Thu Jun 13 05:49:13 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/boot/arm/uart.c	Thu Jun 13 05:48:58 2013
+++ /sys/src/boot/arm/uart.c	Thu Jun 13 05:48:59 2013
@@ -46,7 +46,7 @@
 	Overrun=	1<<5,
 };
 
-Uartregs *uart3regs = UART3REGS;
+Uartregs *uart3regs = (Uartregs*)UART3REGS;
 
 
 /*