Add missing #include of <cctype>, needed for some versions of MinGW.

asio/include/asio/detail/impl/socket_ops.ipp

diff --git a/asio/include/asio/detail/impl/socket_ops.ipp b/asio/include/asio/detail/impl/socket_ops.ipp
index 709530f..6965140 100644
--- a/asio/include/asio/detail/impl/socket_ops.ipp
+++ b/asio/include/asio/detail/impl/socket_ops.ipp
@@ -18,6 +18,7 @@
 #include "asio/detail/config.hpp"
 #include <boost/assert.hpp>
 #include <boost/detail/workaround.hpp>
+#include <cctype>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>