migration/postcopy-ram: Guard use of sys/eventfd.h with CONFIG_EVENTFD

sys/eventfd.h was being guarded only by a check for linux but does
not exist on older distributions like CentOS 5. Move the include
into the code that uses it and add an appropriate guard.

Signed-off-by: Matthew Fortune <matthew.fortune@imgtec.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <6D39441BF12EF246A7ABCE6654B023536BB85DEB@hhmail02.hh.imgtec.org>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
