add _FILE_OFFSET_BITS to pkgconfig file
This was removed in the transition to meson, causing downstream projects to break when built on 32-bit systems.
I'm not sure if just setting it globally is the correct solution. As far as I can tell, meson itself sets it on almost all platforms. There is a get_largefile_args function that sets it, but I can't find any way to access that functionality from a build file.