shill: tighten up constness of PropertyStore

BUG=chromium-os:20543
TEST=unittest

Change-Id: Ifa86f82a883b3c3ec61ce10f8d82564b9a036b04
Reviewed-on: http://gerrit.chromium.org/gerrit/7901
Tested-by: mukesh agrawal <quiche@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
diff --git a/dhcp_config.cc b/dhcp_config.cc
index 3764c19..5561b1e 100644
--- a/dhcp_config.cc
+++ b/dhcp_config.cc
@@ -57,8 +57,8 @@
       task_factory_(this),
       dispatcher_(dispatcher),
       glib_(glib) {
-  store()->RegisterConstString(flimflam::kAddressProperty,
-                               &(properties().address));
+  mutable_store()->RegisterConstString(flimflam::kAddressProperty,
+                                       &(properties().address));
   VLOG(2) << __func__ << ": " << device_name;
 }