spirv-cross.cmake webgpu support

[pgruenbacher]

bgfx added webgpu, this is the main issue I came across when trying to compile with bgfx master.

[pezcode]

@bwrsandman does this fix #78?

[bwrsandman]

Not exactly, WebGPU is a separate 3rd party and should have it's own cmake file.
This here is just adding an include to a spriv tools which is not complete

Also the submodules have not been updated to the webgpu commit.