tools/perf/build: Clean up the libelf logic in config/Makefile

Nest the rules properly. No change in functionality.

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-dDgivr9xtjrof2vmoyOfwxkj@git.kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>