musl removed the LFS64 APIs like mmap64(), which were intended to be a
transitional measure multiple decades ago, causing a build failure
here. Since virtio-gpu sizes and offsets are 64-bit, we do still want
to make sure that we're using 64-bit mmap here, so I've added
-D_FILE_OFFSET_BITS=64, which will ensure that off_t is always 64-bit
in gfxstream guest, and which is generally the modern solution here.
With this change, I am able to build gfxstream with musl.
Fixes: fec8e296a3 ("Make VirtGpu* interfaces")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37086>
6f8cdd8a3c