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>

drivers/isdn/gigaset/asyncdata.c

@@ -16,6 +16,7 @@
 #include "gigaset.h"
 #include <linux/crc-ccitt.h>
 #include <linux/bitrev.h>
+#include <linux/export.h>
 
 /* check if byte must be stuffed/escaped
  * I'm not sure which data should be encoded.