Fix implicit-fallthrough warnings in AVDVirglRenderer.cpp etc.

It will be a global error by default.

Test: make checkbuild
Bug: 112564944
Exempt-From-Owner-Approval: do not block on new warnings
diff --git a/host/commands/emugen/main.cpp b/host/commands/emugen/main.cpp
index 5408617..6200349 100644
--- a/host/commands/emugen/main.cpp
+++ b/host/commands/emugen/main.cpp
@@ -70,7 +70,7 @@
             break;
         case ':':
             fprintf(stderr, "Missing argument !!\n");
-            // fall through
+            [[fallthrough]];
         default:
             usage(argv[0]);
             exit(0);
diff --git a/host/libs/virglrenderer/AVDVirglRenderer.cpp b/host/libs/virglrenderer/AVDVirglRenderer.cpp
index 2be15d5..95e7256 100644
--- a/host/libs/virglrenderer/AVDVirglRenderer.cpp
+++ b/host/libs/virglrenderer/AVDVirglRenderer.cpp
@@ -788,7 +788,7 @@
                 g_last_submit_cmd_ctx->setFence(client_fence_id);
                 break;
             }
-            // Fall through
+            [[fallthrough]];
         default: {
             std::lock_guard<std::mutex> lk(g_fence_deque_mutex);
             g_fence_deque.push_back(client_fence_id);