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/shada.c

@@ -5,6 +5,7 @@
 #include <stdint.h>
 #include <inttypes.h>
 #include <errno.h>
+#include <fcntl.h>
 #ifdef HAVE_UNISTD_H
 # include <unistd.h>
 #endif