serial: 8250: include <linux/serial_reg.h> from serial_8250.h

The header file, include/linux/serial_8250.h, contains references to
UART_LSR_BRK_ERROR_BITS and UART_MSR_ANY_DELTA that are defined in
<linux/serial_reg.h>.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/serial_8250.h

@@ -12,6 +12,7 @@
 #define _LINUX_SERIAL_8250_H
 
 #include <linux/serial_core.h>
+#include <linux/serial_reg.h>
 #include <linux/platform_device.h>
 
 /*