Add the perf_mmap struct to libperf.

The definition is added into:

  include/internal/mmap.h

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

Committer notes:

Remove unnecessary includes from tools/perf/lib/include/internal/mmap.h,
those will be readded as they become necessary, later in the series.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lore.kernel.org/lkml/20190913132355.21634-11-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>