[C++]: add accessor for actingVersion to fixed flyweight to align with message flyweight and to also silence warnings about unused private fields for fixed flyweights.

Author: tmontgomery

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppGenerator.java

@@ -1591,6 +1591,10 @@ private CharSequence generateFixedFlyweightCode(final String className, final in
             "    {\n" +
             "        return m_bufferLength;\n" +
             "    }\n\n" +
+            "    std::uint64_t actingVersion() const SBE_NOEXCEPT\n" +
+            "    {\n" +
+            "        return m_actingVersion;\n" +
+            "    }\n\n" +
             "    static SBE_CONSTEXPR %3$s sbeSchemaId() SBE_NOEXCEPT\n" +
             "    {\n" +
             "        return %4$s;\n" +