package/libxml2: add optional LZMA support

LZMA support is provided by xz, since liblzma is long dead.

Contrary to zlib, liblzma uses pkg-config, so we do not need to add the
full path to libzma.

Reported-by: vdm on IRC
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>