Added xenkernel linux64 makefile PLATFORM_VARIANT.

A solution to this problem:

http://forum.openframeworks.cc/t/failure-compiling-of-on-an-ubuntu-12-04-03-lts-digital-ocean-droplet/13893

To use on a linux64 platform, run


```
$ cd $(OF_ROOT)/openFrameworks/libs/openFrameworksCompiled/project
$ make PLATFORM_VARIANT=xenkernel
```