ruby/io.h: fix rb_io_buffer_t

* include/ruby/io.h (rb_io_buffer_t): fix usage of PACKED_STRUCT().
  it must surround the whole declaration on VC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46892 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

include/ruby/io.h

@@ -51,12 +51,12 @@ extern "C" {
 
 RUBY_SYMBOL_EXPORT_BEGIN
 
-typedef PACKED_STRUCT(struct {
+PACKED_STRUCT(typedef struct {
     char *ptr;                  /* off + len <= capa */
     int off;
     int len;
     int capa;
-}) rb_io_buffer_t;
+} rb_io_buffer_t);
 
 typedef struct rb_io_t {
     FILE *stdio_file;		/* stdio ptr for read/write if available */