Causes build failures on Debian ia64 platform, and per the glibc docs this
is an internal macro for them that should be getting set automatically
as long as we've defined _GNU_SOURCE before including our first glibc
header.

Fix the task/affinity plugin code to define _GNU_SOURCE first, and not
defer this to the affinity.h include (which may come after some glibc
headers have been included, leading to issues finding the CPU_SETSIZE
macro).

Bug 7152.