v4l2object: Don't use mmap64 if off_t is 64-bit

v4l2object: Don't use mmap64 if off_t is 64-bit

The difference between mmap and mmap64 is the type of 'offset' argument.
mmap64 always uses a 64-bit interger as offset, while mmap uses off_t,
whose size can vary on different operating systems or architectures.

However, not all operating systems support mmap64. Fortunately, although
FreeBSD only has mmap, its off_t is always 64-bit regardless of
architectures, so we can simply use mmap when sizeof(off_t) == 8.

https://bugzilla.gnome.org/show_bug.cgi?id=791779