Fix build on g++ version < 10.

[skasperski]

Build failed on test/unit/test_range.cpp because of what seems to be a bug in g++ < 10. It just moves the member declaration to the top of the templates. If it does not cause any other issue, might be worth for increased compatibility.

libpqxx/test/unit/test_range.cxx:1:
/libpqxx/include/pqxx/range.hxx:56:31: error: ‘m_value’ was not declared in this scope; did you mean ‘value’?
   56 |     noexcept(noexcept(value < m_value))
      |                               ^~~~~~~
      |                               value

[jtv]

Weird. Thanks for the fix! I'll add comments so we don't forget to change it back later.