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/if_cscope.c

@@ -11,6 +11,7 @@
 #include <assert.h>
 #include <errno.h>
 #include <inttypes.h>
+#include <fcntl.h>
 
 #include "nvim/vim.h"
 #include "nvim/ascii.h"