perf symbols: Fix missing label symbols

perf symbols: Fix missing label symbols

Label symbols are missing because elf_sec__is_a() fails to find the
section because the section strings do not match the section headers
because the sections headers are from the 'runtime' object and the
sections strings are from the 'symbol source' object.

Fix by getting the section strings from the 'runtime' object so that
they match the section headers.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1406786474-9306-4-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>