um: Export pm_power_off

...modules are using this symbol.
Export it like all other archs to.

Signed-off-by: Richard Weinberger <richard@nod.at>
diff --git a/arch/um/kernel/reboot.c b/arch/um/kernel/reboot.c
index 9bdf67a..b60a9f8 100644
--- a/arch/um/kernel/reboot.c
+++ b/arch/um/kernel/reboot.c
@@ -12,6 +12,7 @@
 #include <skas.h>
 
 void (*pm_power_off)(void);
+EXPORT_SYMBOL(pm_power_off);
 
 static void kill_off_processes(void)
 {