perf symbols: Unexport symbol_type__is_a()

Now this is only used in the symbols.c file, where it will finally
disappear when we remove 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-a9t4d4hfrycczq9vpsk5sr8q@git.kernel.org


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