xen: Fix compilation error reported by Kbuilt

HVC xen earlycon support is temporary solution till the proper way is
found.
Compilation error:
drivers/tty/hvc/hvc_xen.c:625:13: error: redefinition of
'xenboot_write_console'
    static void xenboot_write_console(struct console *console, const
char *string,
   drivers/tty/hvc/hvc_xen.c:590:13: note: previous definition of
'xenboot_write_console' was here
    static void xenboot_write_console(struct console *console, const
char *string,

Signed-off-by: Michal Simek <michal.simek@xilinx.com>