[C++]: Make constant fields static constexpr

Author: abbradar

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

@@ -1172,7 +1172,7 @@ private CharSequence generateConstPropertyMethods(final String propertyName, fin
  {
  return String.format(
  "\n" +
- indent + " %1$s %2$s(void) const\n" +
+ indent + " static SBE_CONSTEXPR const %1$s %2$s(void)\n" +
  indent + " {\n" +
  indent + " return %3$s;\n" +
  indent + " }\n",