[POWERPC] spufs: add marker for destroy_spu_context

The sputrace module contained a reference to a marker for
destroy_spu_context, but this marker did not appear in the code. Fix
this by adding a marker in the function.

Signed-off-by: Julio M. Merino Vidal <jmerino@ac.upc.edu>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>