sys-auth/rtkit: fix 0.13 meson build scripts

sys-auth/rtkit: fix 0.13 meson build scripts

One, they installed rtkitctl into bin rather than, like autotools, into
sbin. Already fixed upstream.

Two, they aborted if the program 'xxd' couldn't be found without
even trying to use a pre-generated header file. I have just submitted a
pull request to fix this upstream as well.

Will stick with Meson though, in order to revert to autotools someone
would have to make it possible not to install tests and I really do not
want to hack at this particular build system any more than I absolutely
have to.

Signed-off-by: Marek Szuba <marecki@gentoo.org>