xwm: Fix fd leak in selection code

The file descriptors we receive in the wayland protocol callbacks are
already O_CLOEXEC, so don't dup them to set O_CLOEXEC.`
diff --git a/src/xwayland/selection.c b/src/xwayland/selection.c
index 735b726..d220569 100644
--- a/src/xwayland/selection.c
+++ b/src/xwayland/selection.c
@@ -135,7 +135,7 @@
 		xcb_flush(wm->conn);
 
 		fcntl(fd, F_SETFL, O_WRONLY | O_NONBLOCK);
-		wm->data_source_fd = fcntl(fd, F_DUPFD_CLOEXEC, fd);
+		wm->data_source_fd = fd;
 	}
 }