Use the correct namespace for tr1 references.

Change-Id: I130bf9a7c811a2ec14425e007108279e60296035

internal/ceres/collections_port.h

diff --git a/internal/ceres/collections_port.h b/internal/ceres/collections_port.h
index e125f3f..88032a5 100644
--- a/internal/ceres/collections_port.h
+++ b/internal/ceres/collections_port.h
@@ -48,10 +48,10 @@
 namespace internal {
 
 template<typename K, typename V>
-struct HashMap : tr1::unordered_map<K, V> {};
+struct HashMap : std::tr1::unordered_map<K, V> {};
 
 template<typename K>
-struct HashSet : tr1::unordered_set<K> {};
+struct HashSet : std::tr1::unordered_set<K> {};
 
 #ifdef _WIN32
 #define GG_LONGLONG(x) x##I64