fadvise is not present on OpenBSD.

It is just a hint, after all, so it is not an error if it is
unavailable.

Author: 3405691582

src/io.c

@@ -2328,8 +2328,11 @@ _dispatch_operation_advise(dispatch_operation_t op, size_t chunk_size)
 		case ESPIPE: break; // fd refers to a pipe or FIFO
 		default: (void)dispatch_assume_zero(err); break;
 	}
+#elif defined(__OpenBSD__)
+	(void)err;
 #else
 #error "_dispatch_operation_advise not implemented on this platform"
+	(void)err;
 #endif // defined(F_RDADVISE)
 #endif // defined(_WIN32)
 }