sandboxing: allow writing to stats/hidserv-stats

Our sandboxing code would not allow us to write to stats/hidserv-stats,
causing tor to abort while trying to write stats.  This was previously
masked by bug#19556.

changes/bug19557

+  o Major bugfixes (sandboxing):
+    - Our sandboxing code would not allow us to write to stats/hidserv-stats,
+      causing tor to abort while trying to write stats.  This was previously
+      masked by bug 19556.  Fixes bug 19557.

src/or/main.c

@@ -3530,6 +3530,7 @@ sandbox_init_filter(void)
     OPEN_DATADIR2_SUFFIX("stats", "exit-stats", ".tmp");
     OPEN_DATADIR2_SUFFIX("stats", "buffer-stats", ".tmp");
     OPEN_DATADIR2_SUFFIX("stats", "conn-stats", ".tmp");
+    OPEN_DATADIR2_SUFFIX("stats", "hidserv-stats", ".tmp");
 
     OPEN_DATADIR("approved-routers");
     OPEN_DATADIR_SUFFIX("fingerprint", ".tmp");