perf tools: Remove stale prototypes from builtin.h

Some, like prune_packed_objects() are clearly git specific, others
don't have implementations and some are used in just one place, make
them static.

Link: http://lkml.kernel.org/n/tip-faj3c5dnttf3hurv4pujut8n@git.kernel.org


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