BGE: Fix for [#35815] "makeScreenshot() relative path not working in Blenderplayer" reported by HG1.

Someone forgot to add relative path expansion (BLI_path_abs) to the Blenderplayer...

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57645 954f8c5b-7b00-dc11-b283-0030488c597c