uml: fix 5.10 build

uml: fix 5.10 build

the 5.10 uml build currently breaks with:

/usr/bin/ld: arch/um/os-Linux/signal.o: in function `sigusr1_handler':
arch/um/os-Linux/signal.c:141: undefined reference to `uml_pm_wake'

But there's an upstream fix for this. Backport the fix
for now but also let upstream know so it finds its way
through the -stable releases.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
(cherry picked from commit 8bea5edf89e57c32b98620540a457441f5f8ddeb)