Blender Git Loki

Cleanup: Fix build warning on windows

MSBuild on windows currently spews a warning about
buildinfo.h_fake not being generated.

For build info we use a non existing file to trigger a
custom_command on every build, which has worked well for
years now, however in recent versions of MSBuild it has
started issuing warnings about files that should be
generated but are not.

CMake is actually aware of this being a problem and states
in the documentation that "If the output of the custom command
is not actually created as a file on disk it should be marked
with the SYMBOLIC source file property."

This change fixes the build warning by properly marking the
buildinfo.h_fake as symbolic resolving the warning.