Do not define static target for `vulkan_backend_lib` (#2042)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2042

## Context

Fixes an internal CI breakage. The problem was the following:

* the `runtime.cxx_library()` wrapper has an arg `define_static_target` which is default `True`
* This will define a static version of the target with the name `vulkan_backend_lib_static`
* The static target dependencies with the `_static` suffix depending on [this logic](https://fburl.com/code/wz7kn4i4)
* Since `//caffe2:torch_vulkan_graph` is begins with `//caffe2`, it will be patched. However, there is no `"//caffe2:torch_vulkan_graph_static"` defined.

For now, disable defining a static target for `vulkan_backend_lib` to fix CI. Will need to think about the best/cleanest way to handle this - ideally there can be more fine-grained control over what dependencies should be patched.

Reviewed By: larryliu0820

Differential Revision: D54073762

fbshipit-source-id: e85d212637773cf77cfa7ee6f5ff2d8c243ee02d