appveyor: Set git core.autocrlf setting to true.

appveyor: Set git core.autocrlf setting to true.

The git core.autocrlf setting defaults to true (ie, all text files get
checked out as CRLF on Windows), except on Appveyor where's set to
"input" (ie, all text files get checked out with the upstream
repository's line endings, which for us typically means LF.)

And this was masking on Appveyor a regression in gen_xmlpool.py
processing t_options.h with CRLF line endings.

This change makes core.autocrlf to be true, which would have enabled to
immediately catch the issue, as seen in
https://ci.appveyor.com/project/jrfonseca/mesa/build/51

Reviewed-by: Roland Scheidegger <sroland@vmware.com>