[gn build] Port 3d0c616

Author: llvmgnsyncbot

llvm/utils/gn/secondary/lldb/tools/lldb-dap/BUILD.gn

@@ -76,6 +76,7 @@ static_library("lib") {
     "RunInTerminal.cpp",
     "SourceBreakpoint.cpp",
     "Transport.cpp",
+    "Variables.cpp",
     "Watchpoint.cpp",
   ]
 }