perf tools: Use kallsyms__is_function()

Replacing equivalent, the equivalent and longer variation:

	 symbol__is_a(type, MAP__FUNCTION);

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: https://lkml.kernel.org/n/tip-9t3dqogher54owfl9o2mir52@git.kernel.org


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