Use explicit break instead of fall-through to break-only case

clang generates a warning if there's no explicit break or fall-through
annotation. The latter would be kind of silly in this case, and not
robust against any future changes turning the fall-through invalid.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10220>

src/amd/compiler/aco_insert_waitcnt.cpp

@@ -706,6 +706,7 @@ void gen(Instruction* instr, wait_ctx& ctx)
       if (instr->opcode == aco_opcode::s_sendmsg ||
           instr->opcode == aco_opcode::s_sendmsghalt)
          update_counters(ctx, event_sendmsg);
+      break;
    }
    default:
       break;