isdn/capi: fix defined but not used warnings

Fix build warnings in drivers/isdn/capi/ when CONFIG_PROC_FS is not
enabled by marking the unused functions as __maybe_unused.

../drivers/isdn/capi/capi.c:1324:12: warning: 'capi20_proc_show' defined but not used [-Wunused-function]
../drivers/isdn/capi/capi.c:1347:12: warning: 'capi20ncci_proc_show' defined but not used [-Wunused-function]
../drivers/isdn/capi/capidrv.c:2454:12: warning: 'capidrv_proc_show' defined but not used [-Wunused-function]
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: isdn4linux@listserv.isdn4linux.de (subscribers-only)
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/capi/capi.c

@@ -9,6 +9,7 @@
  *
  */
 
+#include <linux/compiler.h>
 #include <linux/module.h>
 #include <linux/errno.h>
 #include <linux/kernel.h>
@@ -1321,7 +1322,7 @@ static inline void capinc_tty_exit(void) { }
  * /proc/capi/capi20:
  *  minor applid nrecvctlpkt nrecvdatapkt nsendctlpkt nsenddatapkt
  */
-static int capi20_proc_show(struct seq_file *m, void *v)
+static int __maybe_unused capi20_proc_show(struct seq_file *m, void *v)
 {
 	struct capidev *cdev;
 	struct list_head *l;
@@ -1344,7 +1345,7 @@ static int capi20_proc_show(struct seq_file *m, void *v)
  * /proc/capi/capi20ncci:
  *  applid ncci
  */
-static int capi20ncci_proc_show(struct seq_file *m, void *v)
+static int __maybe_unused capi20ncci_proc_show(struct seq_file *m, void *v)
 {
 	struct capidev *cdev;
 	struct capincci *np;

drivers/isdn/capi/capidrv.c

@@ -9,6 +9,7 @@
  *
  */
 
+#include <linux/compiler.h>
 #include <linux/module.h>
 #include <linux/errno.h>
 #include <linux/kernel.h>
@@ -2451,7 +2452,7 @@ lower_callback(struct notifier_block *nb, unsigned long val, void *v)
  * /proc/capi/capidrv:
  * nrecvctlpkt nrecvdatapkt nsendctlpkt nsenddatapkt
  */
-static int capidrv_proc_show(struct seq_file *m, void *v)
+static int __maybe_unused capidrv_proc_show(struct seq_file *m, void *v)
 {
 	seq_printf(m, "%lu %lu %lu %lu\n",
 		   global.ap.nrecvctlpkt,