Rename version.h to libweston/version.h

This is an installed public header, and without the subdir would surely
conflict with something else.

include/libweston/meson.build is necessary for putting the generated header in
the right subdirectory so that '#include <libweston/version.h>' can work.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>