perf unwind: Provide only forward declarations for pointer types

No need to drag the headers, helps in untangling them and reducing build
time.

Link: http://lkml.kernel.org/n/tip-l8soqph92duyw5jdha0fij8b@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>