fcntl.h: add AT_RECURSIVE from linux v5.2

apply open_tree with OPEN_TREE_CLONE call to the entire subtree, see

  linux commit a07b20004793d8926f78d63eb5980559f7813404
  vfs: syscall: Add open_tree(2) to reference or clone a mount

include/fcntl.h

@@ -104,6 +104,7 @@ int posix_fallocate(int, off_t, off_t);
 #define AT_STATX_SYNC_AS_STAT 0x0000
 #define AT_STATX_FORCE_SYNC 0x2000
 #define AT_STATX_DONT_SYNC 0x4000
+#define AT_RECURSIVE 0x8000
 
 #define FAPPEND O_APPEND
 #define FFSYNC O_SYNC