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/util/crypto.cpp b/src/util/crypto.cpp
index 490feb8..8967523 100644
--- a/src/util/crypto.cpp
+++ b/src/util/crypto.cpp
@@ -4,7 +4,9 @@
  * See COPYING for copyright and distribution information.
  */
 
-#include "util/crypto.hpp"
+#include "common.hpp"
+
+#include "crypto.hpp"
 
 namespace ndn {