opa: fix this statement may fall through [-Werror=implicit-fallthrough=] found by gcc7

Change-Id: I1edb5a3f7c9ac94c2a922058ca54d4b3e9218b0e
Reviewed-on: https://code.wireshark.org/review/20408
Reviewed-by: Adam Goldman <adam.goldman@intel.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>

epan/dissectors/packet-opa.c

@@ -1119,6 +1119,7 @@ static int dissect_opa_9b(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, v
         if (nextHdr != 0x1B) {   /* no BTH following. */
             break;
         }
+        /* FALL THROUGH */
     case 2: /* LOCAL - BTH - Base Transport Header */
         parse_opa_bth(tvb, pinfo, tree, &offset, &opCode);
         bthFollows = TRUE;