Hunspell: fix MODULE_INCLUDEPATH

This was causing compiler errors about a missing <hunspell/hunspell.h>

Change-Id: Ia7d8360026bf3105ff5b1828b0c7365730cb15a4
Reviewed-by: Jarkko Koivikko <jarkko.koivikko@code-q.fi>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>