wlr_output: fix scope for 'now'

'when' points to now that was defined in the if, so compiler could reuse
that memory area by the time 'when' is called

Found through static analysis.

types/wlr_output.c

@@ -486,8 +486,8 @@ bool wlr_output_swap_buffers(struct wlr_output *output, struct timespec *when,
 		pixman_region32_intersect(&render_damage, &render_damage, damage);
 	}
 
+	struct timespec now;
 	if (when == NULL) {
-		struct timespec now;
 		clock_gettime(CLOCK_MONOTONIC, &now);
 		when = &now;
 	}