include: power: include missing header for slist

Include missing header for slist.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>
diff --git a/include/power/power.h b/include/power/power.h
index 8b05df1..8824c96 100644
--- a/include/power/power.h
+++ b/include/power/power.h
@@ -8,6 +8,7 @@
 #define ZEPHYR_INCLUDE_POWER_POWER_H_
 
 #include <zephyr/types.h>
+#include <sys/slist.h>
 #include <power/power_state.h>
 #include <stdbool.h>