meson: Require wayland-scanner of a matching version

We have always built libwayland with the scanner from the same build so
that the generated code and installed headers are exactly up-to-date
with the libwayland version. If libwayland was to use a scanner later
than itself, the scanner might do things that are not available in the
libwayland at hand, leading to a broken build or a broken library
(headers).

Signed-off-by: Matt Turner <mattst88@gmail.com>