Win: Copy the "spirv.h" file for Windows builds (KLUDGE) ...

Win: Copy the "spirv.h" file for Windows builds (KLUDGE) ...

The Linux build requires that "glslang" be built in a directory next to the
Vulkan source.  The Windows build (which doesn't build the Intel driver) does
not (at least not yet).  The new "shader_checker" layer needs the "spirv.h"
file in a "SPIRV" directory.

To kludge around this for now, create a "spirv" directory in the "layers"
directory (that will be ignored on Linux, which looks at the capitalization of
files/dirs) which Windows will use to find the "spirv.h" file in.