windows build: add define to avoid compiler warning

A recent mingw compiler has started to complain on "#warning Please
include winsock2.h before windows.h" unless the magic define is set
first.

Reported by: Vincent Torri
Bug: http://www.libssh2.org/mail/libssh2-devel-archive-2011-10/0064.shtml

src/libssh2_priv.h

@@ -44,7 +44,11 @@
 #include "libssh2_config.h"
 
 #ifdef HAVE_WINDOWS_H
+#ifndef WIN32_LEAN_AND_MEAN
+#define WIN32_LEAN_AND_MEAN
+#endif
 #include <windows.h>
+#undef WIN32_LEAN_AND_MEAN
 #endif
 
 #ifdef HAVE_WS2TCPIP_H