perf scripting python: Avoid declaring function pointers with a visibility attribute

perf scripting python: Avoid declaring function pointers with a visibility attribute

To avoid this:

  util/scripting-engines/trace-event-python.c: In function 'python_start_script':
  util/scripting-engines/trace-event-python.c:1595:2: error: 'visibility' attribute ignored [-Werror=attributes]
   1595 |  PyMODINIT_FUNC (*initfunc)(void);
        |  ^~~~~~~~~~~~~~

That started breaking when building with PYTHON=python3 and these gcc
versions (I haven't checked with the clang ones, maybe it breaks there
as well):

  # export PERF_TARBALL=http://192.168.86.5/perf/perf-5.9.0.tar.xz
  # dm  fedora:33 fedora:rawhide
     1   107.80 fedora:33         : Ok   gcc (GCC) 10.2.1 20201005 (Red Hat 10.2.1-5), clang version 11.0.0 (Fedora 11.0.0-1.fc33)
     2    92.47 fedora:rawhide    : Ok   gcc (GCC) 10.2.1 20201016 (Red Hat 10.2.1-6), clang version 11.0.0 (Fedora 11.0.0-1.fc34)
  #

Avoid that by ditching that 'initfunc' function pointer with its:

    #define Py_EXPORTED_SYMBOL _attribute_ ((visibility ("default")))
    #define PyMODINIT_FUNC Py_EXPORTED_SYMBOL PyObject*

And just call PyImport_AppendInittab() at the end of the ifdef python3
block with the functions that were being attributed to that initfunc.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>