Add B time to linux file accessor

[mgreen27]

Currently we do not collect B time on linux machines that are leveraging the file accessor.

We should implement B time either for the file accessor or a new accessor for relevant linux filesystems.

[mgreen27]

velociraptor/accessors/file/accessor_linux.go

Line 32 in 2c800b1

// On Linux we need xstat() support to get birth time.

[scudette]

This is now available as part of the ext4 accessor.