meson: no need to rebuild on install

On install, Meson will rebuild targets which don't define their
dependencies, as it can't know if they need to be rebuilt or not.

This includes gtk-doc, which can be slow to run.  As we control the
execution of meson we know that a rebuild isn't required, so we can pass
--no-rebuild and speed up the install task.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>