[gn build] Port 1621486

Author: llvmgnsyncbot

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

@@ -21,6 +21,7 @@ static_library("lib") {
   sources = [
     "Breakpoint.cpp",
     "BreakpointBase.cpp",
+    "ClientLauncher.cpp",
     "CommandPlugins.cpp",
     "DAP.cpp",
     "DAPError.cpp",