misdn: Fix a file descriptor leak in _mi_e1_line_update

The socket is created to get information about the ISDN system
and can be released once we have found the information.

Fixes: Coverity CID 1040691
diff --git a/src/input/misdn.c b/src/input/misdn.c
index a72a21e..5966817 100644
--- a/src/input/misdn.c
+++ b/src/input/misdn.c
@@ -640,6 +640,7 @@
 	fprintf(stdout, "        nrbchan:        %d\n", devinfo.nrbchan);
 	fprintf(stdout, "        name:           %s\n", devinfo.name);
 #endif
+	close(sk);
 
 	if (!(devinfo.Dprotocols & (1 << ISDN_P_NT_E1))) {
 		fprintf(stderr, "error: card is not of type E1 (NT-mode)\n");