include/linux/sdla.h: remove the prototype of sdla()

`make headers_check` complains that

linux-2.6/usr/include/linux/sdla.h:116: userspace cannot reference
function or variable defined in the kernel

this is due to that there is no such a kernel function,

void sdla(void *cfg_info, char *dev, struct frad_conf *conf, int quiet);

I don't know why we have it in a kernel header, so remove it.
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sdla.h

@@ -112,11 +112,7 @@ struct sdla_dlci_conf {
    short Tb_max;
 };
 
-#ifndef __KERNEL__
-
-void sdla(void *cfg_info, char *dev, struct frad_conf *conf, int quiet);
-
-#else
+#ifdef __KERNEL__
 
 /* important Z80 window addresses */
 #define SDLA_CONTROL_WND		0xE000