un-hardcode paths in the python tapset

 I'm getting this on my machine now, but I don't know if it is related to those modifications, Ill check tomorrow.

WARNING: kbuild exited with status: 2
semantic error: type definition 'task_struct' not found in 'kernel<linux/sched.h>': operator '@cast' at /home/whirm/venv/inst/share/systemtap/tapset/linux/task.stp:74:26
	        source:     return kernel_string(@cast(task, "task_struct", "kernel<linux/sched.h>")->comm)