Fix HAVE_MUTEX_OWNER_TASK_STRUCT autotools check on PPC64

The HAVE_MUTEX_OWNER_TASK_STRUCT fails on PPC64 with the following
error:

error: 'current' undeclared (first use in this function)

We include linux/sched.h to ensure that current is available.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>