liblas: fix gdal 1.11 and libgeotiff defined type conflict

Fixes error with [gdal 1.11 and libgeotiff defined type conflict for `_GTIFcalloc`](https://gist.github.com/dakcarto/95bf906315e085c7b6e7) that was initially [reported/fixed here](https://github.com/libLAS/libLAS/issues/33) (secondary to original issue).