libiscsi: Port to gcc 7

Avoid that building with gcc 7 fails as follows:

iscsi-dd.c: In function 'cscd_ident_inq':
iscsi-dd.c:405:8: error: this statement may fall through [-Werror=implicit-fallthrough=]
     if (prev_type <= desig->designator_type) {
        ^
iscsi-dd.c:409:4: note: here
    default:
    ^~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>

examples/iscsi-dd.c

@@ -406,6 +406,7 @@ void cscd_ident_inq(struct iscsi_context *iscsi,
 					tgt_desig = desig;
 					prev_type = desig->designator_type;
 				}
+				/* fall through */
 			default:
 				continue;
 		}