PEL: Allow default AdditionalData construction

Enable the default contructor so that an object can be created
without needing any data.

Also make getValue() const as it is const.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: Id81879e0367edb1055cbf848ed738bc8dd86e953
diff --git a/extensions/openpower-pels/additional_data.hpp b/extensions/openpower-pels/additional_data.hpp
index 5cea69b..b855054 100644
--- a/extensions/openpower-pels/additional_data.hpp
+++ b/extensions/openpower-pels/additional_data.hpp
@@ -23,7 +23,7 @@
 class AdditionalData
 {
   public:
-    AdditionalData() = delete;
+    AdditionalData() = default;
     ~AdditionalData() = default;
     AdditionalData(const AdditionalData&) = default;
     AdditionalData& operator=(const AdditionalData&) = default;
@@ -57,7 +57,7 @@
      *
      * @return optional<string> - the value, if found
      */
-    std::optional<std::string> getValue(const std::string& key)
+    std::optional<std::string> getValue(const std::string& key) const
     {
         auto entry = _data.find(key);
         if (entry != _data.end())