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/interest.cpp b/src/interest.cpp
index 6c030dc..da3cab6 100644
--- a/src/interest.cpp
+++ b/src/interest.cpp
@@ -6,6 +6,7 @@
  */
 
 #include "common.hpp"
+
 #include "interest.hpp"
 #include "util/random.hpp"