build: use C++11

Change-Id: I0e58ac4e9cb42d07a9b58125d761875f91c7744c
Refs: #1930
diff --git a/src/storage/sqlite-storage.cpp b/src/storage/sqlite-storage.cpp
index 78b1792..7edcda7 100644
--- a/src/storage/sqlite-storage.cpp
+++ b/src/storage/sqlite-storage.cpp
@@ -25,6 +25,8 @@
 
 namespace repo {
 
+using std::string;
+
 SqliteStorage::SqliteStorage(const string& dbPath)
   : m_size(0)
 {