forward: Add forward declaration for LockFile

src/basic/forward.h

@@ -92,6 +92,7 @@ typedef struct Set Set;
 
 typedef struct dual_timestamp dual_timestamp;
 typedef struct triple_timestamp triple_timestamp;
+typedef struct LockFile LockFile;
 typedef struct PidRef PidRef;
 typedef struct Prioq Prioq;
 typedef struct RateLimit RateLimit;