Add register cache support in HardwareRegister class

Signed-off-by: Zane Shelley <zshelle@us.ibm.com>
Change-Id: Ic671a099fdcd98d1f0158289ef321b968f1908ca
diff --git a/src/hei_includes.hpp b/src/hei_includes.hpp
index 3c29a95..82b38b3 100644
--- a/src/hei_includes.hpp
+++ b/src/hei_includes.hpp
@@ -9,6 +9,7 @@
 // Standard library includes
 #include <stdlib.h>
 #include <stdint.h>
+#include <map>
 #include <vector>
 
 // The user application must define "hei_user_defines.hpp" with the following