parser : implement header check

This change implements a parser method to check if the OpenPOWER VPD has
the mandatory header record, VHDR.

Change-Id: I9e4cebd8f69e47e1ab0f1b569fe4af2c82a5a137
Signed-off-by: Deepak Kodihalli <dkodihal@in.ibm.com>
diff --git a/impl.hpp b/impl.hpp
index 28fcbcc..63128bf 100644
--- a/impl.hpp
+++ b/impl.hpp
@@ -54,6 +54,9 @@
         Store run();
 
     private:
+        /** @brief Checks if the VHDR record is present in the VPD */
+        void checkHeader() const;
+
         /** @brief OpenPOWER VPD in binary format */
         Binary vpd;