libs: force lzma_wrapper to be retained

When CONFIG_SQUASHFS=m and CONFIG_DECOMPRESS_LZMA=m, decompress_lzma
is built but then discarded from the library because no built-in code
uses it, so change it from a lib- to an obj- to force it to be kept
in the library.

ERROR: "unlzma" [fs/squashfs/squashfs.ko] undefined!

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Phillip Lougher <phillip@lougher.demon.co.uk>
Cc: Michal Marek <mmarek@suse.cz>