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>

arch/um/sys-x86_64/signal.c

@@ -6,6 +6,7 @@
 
 #include <linux/personality.h>
 #include <linux/ptrace.h>
+#include <linux/kernel.h>
 #include <asm/unistd.h>
 #include <asm/uaccess.h>
 #include <asm/ucontext.h>