Build fix for MacOS and Fuchsia
Build fix for MacOS and Fuchsia

There was linux specific code in Vulkan's BUILD.gn file.

Change-Id: I22b8cd0b97bd6acfb545e582c6e1dc784f00b2cb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32388
Presubmit-Ready: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Kokoro-Presubmit: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>

google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader