Windows: Include <fcntl.h> for file constants.

This header is required by POSIX for the constants (O_RDONLY, etc.)
but we were only including it on Unix systems as a side effect of
including <unistd.h>.

src/nvim/undo.c

@@ -77,6 +77,7 @@
 #include <limits.h>
 #include <stdbool.h>
 #include <string.h>
+#include <fcntl.h>
 
 #include "nvim/vim.h"
 #include "nvim/ascii.h"