[gn build] Port df6f756

Author: llvmgnsyncbot

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

@@ -51,6 +51,7 @@ executable("lldb-dap") {
     "ProgressEvent.cpp",
     "RunInTerminal.cpp",
     "SourceBreakpoint.cpp",
+    "Watchpoint.cpp",
     "lldb-dap.cpp",
   ]
 }