cairo-util: Don't anger png.h by including setjmp.h

Older libpngs are a bit fussy about this, apparently.
diff --git a/clients/cairo-util.c b/clients/cairo-util.c
index 02cd5bd..bdad696 100644
--- a/clients/cairo-util.c
+++ b/clients/cairo-util.c
@@ -25,7 +25,6 @@
 #include <string.h>
 #include <stdio.h>
 #include <math.h>
-#include <setjmp.h>
 #include <cairo.h>
 #include "cairo-util.h"