Fix -Wimplicit-fallthrough warnings in netd

Test: atest netd_integration_test
Change-Id: Ib5ab9661fb429fdccd6a4c4b5893831ff5fe104e
diff --git a/server/XfrmController.cpp b/server/XfrmController.cpp
index f915814..4574313 100644
--- a/server/XfrmController.cpp
+++ b/server/XfrmController.cpp
@@ -560,7 +560,7 @@
             // direction is ultimately input.
             saInfo.encap.srcPort = encapLocalPort;
             saInfo.encap.dstPort = encapRemotePort;
-        // fall through
+            [[fallthrough]];
         case XfrmEncapType::NONE:
             saInfo.encap.type = static_cast<XfrmEncapType>(encapType);
             break;