Update on "[ET-VK] Fix OSS build + separate test build into its own CMakeLists.txt"

## Context

As title. In the next diff, the vulkan test binary will be added to CI.

Differential Revision: [D57747739](https://our.internmc.facebook.com/intern/diff/D57747739)

[ghstack-poisoned]

Author: SS-JIA

backends/vulkan/main.cpp

@@ -26,6 +26,7 @@ def define_common_targets():
         visibility = [
             "//executorch/runtime/core/exec_aten/...",
             "//executorch/runtime/core/portable_type/test/...",
+            "//executorch/backends/...",
         ],
         exported_deps = [
             ":scalar_type",