isdn: kcapi: don't build unused procfs code

The procfs file is completely unused without CONFIG_PROC_FS but causes
a compile time warning:

drivers/isdn/capi/kcapi_proc.c:97:36: error: unused variable 'seq_controller_ops' [-Werror,-Wunused-const-variable]
static const struct seq_operations seq_controller_ops = {
drivers/isdn/capi/kcapi_proc.c:104:36: error: unused variable 'seq_contrstats_ops' [-Werror,-Wunused-const-variable]
drivers/isdn/capi/kcapi_proc.c:179:36: error: unused variable 'seq_applications_ops' [-Werror,-Wunused-const-variable]
drivers/isdn/capi/kcapi_proc.c:186:36: error: unused variable 'seq_applstats_ops' [-Werror,-Wunused-const-variable]

Remove the file from the build in that config and make the calls into
it conditional instead.

Signed-off-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/isdn/capi/Makefile

···
 # Makefile for the CAPI subsystem used by BT_CMTP
 
 obj-$(CONFIG_BT_CMTP)			+= kernelcapi.o
-kernelcapi-y				:= kcapi.o capiutil.o capi.o kcapi_proc.o
+kernelcapi-y				:= kcapi.o capiutil.o capi.o
+kernelcapi-$(CONFIG_PROC_FS)		+= kcapi_proc.o

drivers/isdn/capi/kcapi.c

···
 		return err;
 	}
 
-	kcapi_proc_init();
+	if (IS_ENABLED(CONFIG_PROC_FS))
+		kcapi_proc_init();
+
 	return 0;
 }
 
 void kcapi_exit(void)
 {
-	kcapi_proc_exit();
+	if (IS_ENABLED(CONFIG_PROC_FS))
+		kcapi_proc_exit();
 
 	cdebug_exit();
 	destroy_workqueue(kcapi_wq);