nspawn: define MS_MOVE manually if needed

src/missing.h

@@ -122,4 +122,8 @@ struct btrfs_ioctl_vol_args {
 #define BTRFS_SUPER_MAGIC 0x9123683E
 #endif
 
+#ifndef MS_MOVE
+#define MS_MOVE 8192
+#endif
+
 #endif

src/nspawn.c

@@ -36,6 +36,7 @@
 
 #include "log.h"
 #include "util.h"
+#include "missing.h"
 
 static char *arg_directory = NULL;