io.c (struct copy_stream_struct): packing

Reduce the struct to 80 bytes (from 88) on amd64 to reduce
stack use.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66049 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

io.c

@@ -10697,11 +10697,11 @@ struct copy_stream_struct {
 
     int src_fd;
     int dst_fd;
-    int close_src;
-    int close_dst;
+    unsigned close_src : 1;
+    unsigned close_dst : 1;
+    int error_no;
     off_t total;
     const char *syserr;
-    int error_no;
     const char *notimp;
     VALUE th;
 };