build: Experimental support to build using precompiled headers

This type of build needs to be explicitly enabled using --with-pch
configure option.

Change-Id: I02dc304187d54c3cf40f1d51b1b11b21f868e225
diff --git a/src/common.cpp b/src/common.cpp
index c3dedd2..7665dc8 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -5,8 +5,8 @@
  * See COPYING for copyright and distribution information.
  */
 
-#include <sstream>
 #include "common.hpp"
+
 #include "util/time.hpp"
 
 using namespace std;