[gn] fix typo in 8c56e78

The missing trailing comma confuses the sync script.

Author: nico

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

@@ -52,6 +52,6 @@ executable("lldb-dap") {
     "RunInTerminal.cpp",
     "SourceBreakpoint.cpp",
     "lldb-dap.cpp",
-    "Watchpoint.cpp"
+    "Watchpoint.cpp",
   ]
 }