Move inappropriate placement of ckpool definition from libckpool header

src/ckpool.h

@@ -52,6 +52,8 @@ struct ckmsgq {
 
 typedef struct ckmsgq ckmsgq_t;
 
+typedef struct proc_instance proc_instance_t;
+
 struct proc_instance {
 	ckpool_t *ckp;
 	unixsock_t us;

src/libckpool.h

@@ -309,9 +309,6 @@ struct unixsock {
 
 typedef struct unixsock unixsock_t;
 
-typedef struct proc_instance proc_instance_t;
-
-
 void _json_check(json_t *val, json_error_t *err, const char *file, const char *func, const int line);
 #define json_check(VAL, ERR) _json_check(VAL, ERR,  __FILE__, __func__, __LINE__)