Mark waffle-jna dependency optional in module descriptor

As discussed in CONJ-1025, the dependency on waffle-jna is used only for the GSSAPI feature on Windows. Clients who want a smaller jar size can exclude the dependency using Maven or Gradle configuration.

*JPMS*
Excluding the dependency on waffle-jna works, – most of the time. However, the `module-info` of mariadb-java-client uses `requires waffle.jna`. This means, with JPMS enabled, waffle-jna must be present.

The optionality of the waffle-jna dependency should be reflected in the JPMS module descriptor. The requires declaration should be requires static to properly reflect that waffle-jna is optional.

It used to be `requires static`, but this commit changed it to `requires`. I'm not sure why. https://github.com/mariadb-corporation/mariadb-connector-j/commit/5ac67b2d17dcf4e0272781426d67299dedf5a377