perf buildid-list: We need to check if a file is ELF first

I.e. before we try to use it as a perf.data file by calling
perf_session__new, otherwise we lose the feature that shows the
build id for the given ELF file, this one:

[root@sandy redhat-perfdata-mtech-15]# perf buildid-list -i /root/.debug/.build-id/97/54896de655b6ac088ec2bf5113b35c06f72709
9754896de655b6ac088ec2bf5113b35c06f72709
[root@sandy redhat-perfdata-mtech-15]# perf buildid-list -i /lib/libc-2.12.so
38adaeff4f7c21899b13b28c1a2e6c199ca4c744
[root@sandy redhat-perfdata-mtech-15]#

Regression introduced in:

efad1415 "perf report: Accept fifos as input file"

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Robert Richter <robert.richter@amd.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-3ktgyg83fwpqyfpoj0t2ezp0@git.kernel.org


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