tools lib symbols: Introduce kallsyms__is_function()

Out of symbol_type__is_a(type, MAP__FUNCTION), which is the only variant
used so far, useful in a kallsyms library and one more step in ditching
the MAP__FUNCTION/VARIABLE split.

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-faonqs76n5808z9mq77edr94@git.kernel.org


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