vsnprintf.c: constify

* vsnprintf.c (FILE::vextra): constify the return value.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51965 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

sprintf.c

@@ -1325,7 +1325,7 @@ ruby__sfvwrite(register rb_printf_buffer *fp, register struct __suio *uio)
     return 0;
 }
 
-static char *
+static const char *
 ruby__sfvextra(rb_printf_buffer *fp, size_t valsize, void *valp, long *sz, int sign)
 {
     VALUE value, result = (VALUE)fp->_bf._base;

vsnprintf.c

@@ -177,7 +177,7 @@ typedef	struct __sFILE {
 	struct	__sbuf _bf;	/* the buffer (at least 1 byte, if !NULL) */
 	size_t	_lbfsize;	/* 0 or -_bf._size, for inline putc */
 	int	(*vwrite)(/* struct __sFILE*, struct __suio * */);
-	char	*(*vextra)(/* struct __sFILE*, size_t, void*, long*, int */);
+	const char *(*vextra)(/* struct __sFILE*, size_t, void*, long*, int */);
 } FILE;