meson: fix seatdpath with absolute bindir

Quoting the Meson documentation:

> Note that the value returned for built-in options that end in `dir`
> such as `bindir` and `libdir` is usually a path relative to (and
> inside) the `prefix` but you should not rely on that, as it can also
> be an absolute path [in some cases](Builtin-options.md#universal-options).
> [`install_dir` arguments](Installing.md) handle that as expected
> but if you need an absolute path, e.g. to use in a define etc.,
> you should use the path concatenation operator like this:
> `get_option('prefix') / get_option('localstatedir')`.
> Never manually join paths as if they were strings.

The concatenation of two absolute paths caused seatd-launch in Nixpkgs
to try to launch e.g.
/nix/store/43wyk9s2l2z8cparnshbf24d39vm5272-seatd-0.7.0//nix/store/j9a7k4qqwc3byyfmpqwg46shmh6g82yf-seatd-0.7.0-bin/bin/seatd.

meson.build

@@ -22,7 +22,7 @@ if defaultpath == ''
 	endif
 endif
 
-seatdpath = '@0@/@1@/seatd'.format(get_option('prefix'), get_option('bindir'))
+seatdpath = get_option('prefix') / get_option('bindir') / 'seatd'
 
 cc = meson.get_compiler('c')