textproc/py-mkdocs-include-markdown-plugin: Update 7.1.5 => 7.1.6

Changelog:

• Fix internal anchor in included file incorrectly rewritten

https://github.com/mondeja/mkdocs-include-markdown-plugin/releases/tag/v7.1.6

• Also add platformdir as a port dependency. platformdir is an optional dependency in upstream, but mkdocs already depends on it, so this just makes use of dependency explicit.

PR: 287550