This fixes compilation issues when btrfs.h isn't available (because we just need the relevant structs, which for userspace programs are supposed to come from btrfs-progs instead of the kernel headers). Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon) Upstream-commit: 6922f1be08111d889b0585b763b08f92d7a55e05 Component: engine