Update EGLNativeTypeBoxee.cpp

[cstdstring] demise Format, replacing with StringUtils::Format

Wouldn't build without these changes. I expect there may be more changes to follow in the next commits.

XBMC team are doing a lot of cstdstring changes to existing code.
http://forum.xbmc.org/showthread.php?tid=176570
https://github.com/xbmc/xbmc/tree/master/xbmc/windowing/egl

Seems to build fine and running well as far as I can see, requires further testing.