Add perf_thread_map struct to libperf.

It's added as a declaration into into:

  include/perf/threadmap.h

which will be included by users.

The perf_thread_map struct definition is added into:

  include/internal/threadmap.h

which is not to be included by users, but shared within perf and
libperf.

We tried the total separation of the perf_thread_map struct in libperf,
but it lead to complications and much bigger changes in perf code, so we
decided to share the declaration.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190721112506.12306-32-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>