Summarry pretty much says it all. if you unpack teh ebuild you will find the file on the source code @ /var/tmp/portage/guichan-0.4.0/work/guichan-0.4.0/include/guichan/sdl/ but the file is not installed when the ebuild is emerged. Workaround: manually unpack teh ebuild and copy the file.
Fixed in 0.4.0-r1, thanks for report