shm_open is a POSIX function creating an in-memory file. Using it simplifies
the code and removes the dependency on XDG_RUNTIME_DIR. The only downside is
that we need to generate a random name for the shm file.
* Moved os-compatibility.c to util
* Added header under util
* Removed static since it isn't needed (i think so)
* Adjusted meson.build to include lib_wlr
Improved some codestyle
* Added guard to os-compatibility.h
* Fixed typo in include statment
Adjusted Guard
* Changed guard to _WLR_UTIL_OS_COMPATIBILITY
