Include stddef.h

I couldn't install the gem because of compiler errors, which are the same ones as described in this issue on Google Code: https://code.google.com/p/ttf2eot/issues/detail?id=29

The fix in this pull request is the same as the one in the issue. All credit goes to Maksym Sditanov, who wrote the original patch.

Also: I only tested this on Ubuntu Trusty 14.04.