builtin_read: pickup MB_CUR_MAX from stdlib not xlocale

Fixes building on OpenBSD; work on #4184.

(cherry picked from commit 874a675e7f)

src/builtin_read.cpp

@@ -4,10 +4,10 @@
 #include <errno.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>