[physfs] Fix compilation on OS X 10.7

[Ryan C. Gordon]

> The fix is here:
> 
> https://trac.macports.org/attachment/ticket/56077/patch-physfs-objectatindex.diff

This patch is now https://hg.icculus.org/icculus/physfs/rev/46561a309895 
for the development branch and 
https://hg.icculus.org/icculus/physfs/rev/baf15f9e8d9a for the 
stable-3.0 branch, thanks!

(if you or Ken want your name in the docs/CREDITS.txt file, let me know!)

--ryan.