UML: add kernel.h include to signal.c

Commit 814f75ec ("UML: Drop private round_down definition") dropped
the private round_down() macro and makes signal.c use the one from kernel.h
instead.

kernel.h is implicitly included through uaccess.h -> sched.h -> kernel.h
but we'd rather make this dependency explicit.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>