add missing build dependency for onnxruntime_providers_webgpu (#23324)

### Description

Fixes build when specify with flag `--target
onnxruntime_providers_webgpu`

Otherwise the following error will occur:
```
  range.cc
D:\code\onnxruntime\build\Windows\Debug\_deps\onnx-src\onnx\onnx_pb.h(65,10): error C1083: Cannot open include file: 'o
nnx/onnx-ml.pb.h': No such file or directory [D:\code\onnxruntime\build\Windows\Debug\onnxruntime_providers_webgpu.vcxp
roj]
  (compiling source file '../../../onnxruntime/core/providers/webgpu/math/binary_elementwise_ops.cc')
```

cmake/onnxruntime_providers_webgpu.cmake

@@ -61,4 +61,5 @@
       VERBATIM )
   endif()
 
+  add_dependencies(onnxruntime_providers_webgpu ${onnxruntime_EXTERNAL_DEPENDENCIES})
   set_target_properties(onnxruntime_providers_webgpu PROPERTIES FOLDER "ONNXRuntime")