kmemtrace, kbuild: fix slab.h dependency problem in lib/decompress_unlzma.c

Impact: cleanup

lib/decompress_unlzma.c depends on slab.h without including it:

    CC      lib/decompress_unlzma.o
  lib/decompress_unlzma.c: In function ‘rc_free’:
  lib/decompress_unlzma.c:122: error: implicit declaration of function ‘kfree’
  lib/decompress_unlzma.c: In function ‘unlzma’:
  lib/decompress_unlzma.c:551: error: implicit declaration of function ‘kmalloc’
  lib/decompress_unlzma.c:551: warning: assignment makes pointer from integer without a cast
  make[1]: *** [lib/decompress_unlzma.o] Error 1
  make: *** [lib/] Error 2

It gets included implicitly currently - but this will not be the
case with upcoming kmemtrace changes.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
LKML-Reference: <1237886521.25315.58.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>