drivers/isdn: Add export.h for EXPORT_SYMBOL/THIS_MODULE as required.

In preparation of the module.h usage cleanup, call out the header
that we need to get EXPORT_SYMBOL variants and THIS_MODULE into scope.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/drivers/isdn/gigaset/i4l.c b/drivers/isdn/gigaset/i4l.c
index 9bec8b9..04231cb 100644
--- a/drivers/isdn/gigaset/i4l.c
+++ b/drivers/isdn/gigaset/i4l.c
@@ -15,6 +15,7 @@
 
 #include "gigaset.h"
 #include <linux/isdnif.h>
+#include <linux/export.h>
 
 #define SBUFSIZE	4096	/* sk_buff payload size */
 #define TRANSBUFSIZE	768	/* bytes per skb for transparent receive */