run: Allow --filesystem=foo to expose sockets

We already kind of support this by exposing the parent directory,
so this isn't really less secure, and you should not be able to
access it unless your user is allowed to anyway.

common/flatpak-run.c

@@ -2295,7 +2295,8 @@ add_expose_path (GHashTable *hash_table,
 
   if (S_ISDIR (st.st_mode) ||
       S_ISREG (st.st_mode) ||
-      S_ISLNK (st.st_mode))
+      S_ISLNK (st.st_mode) ||
+      S_ISSOCK (st.st_mode))
     {
       FlatpakFilesystemMode old_mode;