fix missing function declarations for __stdio_exit

src/stdio/__toread.c

@@ -16,6 +16,8 @@ int __toread(FILE *f)
 static const int dummy = 0;
 weak_alias(dummy, __towrite_used);
 
+void __stdio_exit(void);
+
 void __seek_on_exit()
 {
 	if (!__towrite_used) __stdio_exit();

src/stdio/__towrite.c

@@ -19,6 +19,8 @@ int __towrite(FILE *f)
 
 const int __towrite_used = 1;
 
+void __stdio_exit(void);
+
 void __flush_on_exit()
 {
 	__stdio_exit();