Simplify rc_workaround.py

• Per !93 (comment 2015643), the first directory RC searches for includes is the current directory.
• rc_workaround.py places the patched .rc file in the build dir.
• The desired config.h is in the build dir.
• Ergo, there's actually no patching necessary, simply copying the file already makes it use the right config.h.

The patching actually broke building when using build directories nested >= two levels deep. Replacing the patch with a copy fixes this issue.