Use PROJECT_INCLUDE_DIR in ign_auto_headers.hh.in

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

Author: scpeters

cmake/ign_auto_headers.hh.in

@@ -19,5 +19,5 @@
 // This file is automatically generated by CMake. Changes should instead be
 // made to cmake/ign_auto_headers.hh.in in ignition-cmake
 
-#include <ignition/@IGN_DESIGNATION@/config.hh>
+#include <@PROJECT_INCLUDE_DIR@/config.hh>
 ${ign_headers}