liblzma: Avoid multiple definitions of lzma_coder structures.

Only one definition was visible in a translation unit.
It avoided a few casts and temp variables but seems that
this hack doesn't work with link-time optimizations in compilers
as it's not C99/C11 compliant.

Fixes:
http://www.mail-archive.com/xz-devel@tukaani.org/msg00279.html