git: add Ccache folder to .gitignore

It may sound strange, but it is possible to use, with minor adaptations, the Windows CI build script to compile GIMP locally more easily, and this script uses the useful Ccache. (It is also possible to use Ccache without the script and on other systems too.)

However, there is no exception for that in .gitignore, which, due to the numerous files generated, makes the workflow messy: the unstagged changes become unreadable and the IDE (eg VS Code) search engine loses perfomance.

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This MR proposes to add the _ccache folder (the naming used by the CI script, at least) to the .gitignore file to solve these problems that can affect any local compiler using Ccache.