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>

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)
 {