um: Fix build w/o CONFIG_PM_SLEEP

uml_pm_wake() is unconditionally called from the SIGUSR1 wakeup
handler since that's in the userspace portion of UML, and thus
a bit tricky to ifdef out. Since pm_system_wakeup() can always
be called (but may be an empty inline), also simply always have
uml_pm_wake() to fix the build.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>