perf tools: Be consistent on the type of map->symbols[] interator

In a few cases we were using 'enum map_type' and that triggered this
warning when using clang:

  util/session.c:1923:16: error: comparison of constant 2 with expression of type 'enum map_type' is always true
      [-Werror,-Wtautological-constant-out-of-range-compare]
        for (i = 0; i < MAP__NR_TYPES; ++i) {

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-i6uyo6bsopa2dghnx8qo7rri@git.kernel.org


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