Add FreeBSD compatible input.h and uinput.h

Add FreeBSD compatible input.h and uinput.h

Add FreeBSD compatible input.h and uinput.h files.
This is done by moving the linux files to include/linux/linux, adding
the freebsd versions in include/linux/freebsd, and then changing
include/linux/[u]input.h to pull in the right one depending on which OS
we are compiling on.
Make sure that the build infrastructure in meson.build and
autoconf.ac/Makefile.am uses the correct files when building and as
dependency for targets, and ensure that make-event-names.py get the
correct files as arguments.

A similar change has been done in libinput in
61f3e3854458c556a01fb05d7abb22733fd2b7c1

Signed-off-by: Niclas Zeising <zeising@daemonic.se>