Windows: include <io.h>.

if_csope.c uses _open_osfhandle so include the necessary header.
Futher we need <io.h> for read/write/close/lseek.

See: https://msdn.microsoft.com/en-us/library/bdts1c9x.aspx

src/nvim/os/win_defs.h

@@ -3,6 +3,7 @@
 
 #include <windows.h>
 #include <sys/stat.h>
+#include <io.h>
 #include <stdio.h>
 
 #define NAME_MAX _MAX_PATH