src/libudev/missing.h: clean up unused BTRFS_* decls

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
rules: bring up to date

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
docs/gudev/gudev-sections.txt: add two missing functions

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>