adjust fopencookie structure tag for ABI-compat

stdio types use the struct tag names from glibc libio to match C++
ABI.

include/stdio.h

@@ -188,7 +188,7 @@ typedef ssize_t (cookie_write_function_t)(void *, const char *, size_t);
 typedef int (cookie_seek_function_t)(void *, off_t *, int);
 typedef int (cookie_close_function_t)(void *);
 
-typedef struct {
+typedef struct _IO_cookie_io_functions_t {
 	cookie_read_function_t *read;
 	cookie_write_function_t *write;
 	cookie_seek_function_t *seek;