builtin_read: pickup MB_CUR_MAX from stdlib not xlocale

Fixes building on OpenBSD; work on #4184.

src/builtin_read.cpp

@@ -5,10 +5,10 @@
 #include <limits.h>
 #include <stddef.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 #include <wchar.h>
-#include <xlocale.h>
 
 #include <algorithm>
 #include <memory>