move __stdio_exit_needed to stdio_impl.h

this functions is glue for linking dependency logic.

src/stdio/__toread.c

@@ -13,8 +13,6 @@ int __toread(FILE *f)
 	return (f->flags & F_EOF) ? EOF : 0;
 }
 
-void __stdio_exit_needed(void);
-
 void __toread_needs_stdio_exit()
 {
 	__stdio_exit_needed();

src/stdio/__towrite.c

@@ -17,8 +17,6 @@ int __towrite(FILE *f)
 	return 0;
 }
 
-void __stdio_exit_needed(void);
-
 void __towrite_needs_stdio_exit()
 {
 	__stdio_exit_needed();