Add support for win32.

Edit win32/config.mk to set the paths to your openssl and zlib headers and
libraries.

Then, from the root of the tree:

	nmake -f NMakefile

voila.

src/channel.c

@@ -36,7 +36,9 @@
  */
 
 #include "libssh2_priv.h"
+#ifndef WIN32
 #include <unistd.h>
+#endif
 
 /* {{{ libssh2_channel_nextid
  * Determine the next channel ID we can use at our end