perf buildid: Rename and export build_id_cache__cachedir()

Rename and export build_id_cache__cachedir() for retrieving use of the
path of cache directory for given build_id.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Hemant Kumar <hemant@linux.vnet.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20160608092930.3116.67575.stgit@devbox


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