Log Message:
libmaxminddb: update to 1.6.0.

## 1.6.0 - 2021-04-29

* This release includes several improvements to the CMake build. In
  particular:
  * C99 support is now properly enabled, fixing builds on older `gcc`
    versions. Pull request by Jan Vcelak. GitHub #257.
  * `CMAKE_SHARED_LIBRARY_PREFIX` and `CMAKE_STATIC_LIBRARY_PREFIX` are
    no longer explicitly set and now use the default values for the platform.
    Pull request by Jan Vcelak. GitHub #258.
  * `target_include_directories` now works as expected. Pull request by Jan
    Vcelak. GitHub #259.
  * DLLs are now installed on Windows when `libmaxminddb` is built as a
    shared library. Pull request by Jan Vcelak. GitHub #261.
  * When built as a dynamic library on Windows, all symbols are now exported.
    Pull request by Jan Vcelak. GitHub #262.