meson: use `/` operator instead of `join_paths`
What does this MR do and why?
meson: use /
operator instead of join_paths
Meson does some special handling of /
which allows it to know that the
root path came from a subproject variable, while it doesn't do that for
join_paths
. This means if wayland-protocols comes from a subproject
using join_paths
will cause Meson to think a sandbox violation is
happening, while using /
does not.
fixes: f7da5e3e ("wayland: enable use of wayland-protocols as a subproject")