meson: use a/b instead of join_paths(a,b)

It is nicer and shorter.

tmpfiles.d/meson.build

@@ -52,6 +52,5 @@ endforeach
 
 if enable_tmpfiles and install_sysconfdir
         meson.add_install_script(
-                'sh', '-c',
-                mkdir_p.format(join_paths(sysconfdir, 'tmpfiles.d')))
+                'sh', '-c', mkdir_p.format(sysconfdir / 'tmpfiles.d'))
 endif