Plugins: Use `include_once` instead of `include` in `uninstall_plugin()`, in...

Plugins: Use `include_once` instead of `include` in `uninstall_plugin()`, in case the plugin file is already included.

Props valchovski, pcfreak30.
Fixes #47796.
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git-svn-id: https://core.svn.wordpress.org/trunk@48095 1a063a9b-81f0-0310-95a4-ce76da25c4cd