Define _xcb_map_new with explicit void arg list instead of empty one

Fixes Solaris Studio compiler warning:
"xcb_list.c", line 50: warning: old style function definition

and gcc warning:
xcb_list.c: In function '_xcb_map_new':
xcb_list.c:50:11: warning: old-style function definition [-Wold-style-definition]

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

src/xcb_list.c

@@ -47,7 +47,7 @@ struct _xcb_map {
 
 /* Private interface */
 
-_xcb_map *_xcb_map_new()
+_xcb_map *_xcb_map_new(void)
 {
     _xcb_map *list;
     list = malloc(sizeof(_xcb_map));