kmemtrace, squashfs: fix slab.h dependency problem in squasfs

Impact: cleanup

fs/squashfs/export.c depends on slab.h without including it:

    CC      fs/squashfs/export.o
  fs/squashfs/export.c: In function ‘squashfs_read_inode_lookup_table’:
  fs/squashfs/export.c:133: error: implicit declaration of function ‘kmalloc’
  fs/squashfs/export.c:133: warning: assignment makes pointer from integer without a cast
  fs/squashfs/export.c:143: error: implicit declaration of function ‘kfree’
  make[1]: *** [fs/squashfs/export.o] Error 1
  make: *** [fs/squashfs/] 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: <1237884999.25315.41.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>