build: Fix build when using -fno-inline

build: Fix build when using -fno-inline

struct task_slice.migrated is not initialised by this function, and
subsequently returned by value, leading to the error:

sched_sedf.c: In function ‘sedf_do_extra_schedule’:
sched_sedf.c:711: error: ‘ret.migrated’ may be used uninitialised in
this function

for both gcc 4.1.2 and 4.4.3 (which are the two I have easily to hand)
when combined with the -fno-inline compile option.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Committed-by: Keir Fraser <keir@xen.org>