meson: use join_paths consistently

With -Dsplit-usr=true, we set rootprefix to /. This leads to //lib/systemd or
//lib/udev for various dir variables. Using join_paths() avoids this.

shell-completion/bash/meson.build

@@ -6,7 +6,7 @@ if bashcompletiondir == ''
   if bash_completion.found()
     bashcompletiondir = bash_completion.get_pkgconfig_variable('completionsdir')
   else
-    bashcompletiondir = datadir + '/bash-completion/completions'
+    bashcompletiondir = join_paths(datadir, 'bash-completion/completions')
   endif
 
   message('bash completions: @0@'.format(bashcompletiondir))