compat: define PM_EVENT_SLEEP only if not defined

compat: define PM_EVENT_SLEEP only if not defined

This fixes a compilation warning on Ubuntu's 2.6.24
kernel which added the PM_EVENT_SLEEP even though upstream
Linux 2.6.24.7 (the last 2.6.24) does not have it. Its
a trivial change and I'm willing to support it to avoid
compilation.

Tested against /lib/modules/2.6.24-27-lpia/build

These kernel headers can be foudn here:

http://kernel.org/pub/linux/kernel/people/mcgrof/ubuntu/debs/kernel-headers/

Install with dpkg -i --force-architecture *.deb

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>