Add a .mailmap file

This will let command-line Git tools re-map my name and e-mail address properly.
I'm using my personal e-mail address and not my Collabora address because I'm
not actively contributing to Wayland anymore and this is mostly for letting
people find me should they dig me up in the project history.

Signed-off-by: Faith Ekstrand <faith@gfxstrand.net>