Bugfix: Declare manual_enable_pool in miner.h, for api.c use

miner.h

@@ -1522,6 +1522,7 @@ extern void kill_work(void);
 extern int prioritize_pools(char *param, int *pid);
 extern void validate_pool_priorities(void);
 extern void enable_pool(struct pool *);
+extern void manual_enable_pool(struct pool *);
 extern void disable_pool(struct pool *, enum pool_enable);
 extern void switch_pools(struct pool *selected);
 extern void remove_pool(struct pool *pool);