NFC: Fix SE API related sparse warning

se_io_cb can be declared static. This fixes the following sparse
warning:

net/nfc/netlink.c:1287:6: warning: symbol 'se_io_cb' was not declared.
Should it be static?

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

net/nfc/netlink.c

@@ -1284,7 +1284,7 @@ struct se_io_ctx {
 	u32 se_idx;
 };
 
-void se_io_cb(void *context, u8 *apdu, size_t apdu_len, int err)
+static void se_io_cb(void *context, u8 *apdu, size_t apdu_len, int err)
 {
 	struct se_io_ctx *ctx = context;
 	struct sk_buff *msg;