Add default Signature constructor for STL functions

Signed-off-by: Zane Shelley <zshelle@us.ibm.com>
Change-Id: Iddf8476537564586215ed236b6072d1c8c399592
diff --git a/src/hei_signature.hpp b/src/hei_signature.hpp
index 5fd2792..b820600 100644
--- a/src/hei_signature.hpp
+++ b/src/hei_signature.hpp
@@ -17,6 +17,9 @@
 class Signature
 {
   public: // Constructors, destructor, assignment, etc.
+    /** @brief Default constructor. */
+    Signature() = default;
+
     /**
      * @brief Constructor from components.
      * @param i_chip     The chip containing this register.