perf tools: Fix python extension build for gcc 8

perf tools: Fix python extension build for gcc 8

The gcc 8 compiler won't compile the python extension code with the
following errors (one example):

  python.c:830:15: error: cast between incompatible  function types from              \
  ‘PyObject * (*)(struct pyrf_evsel *, PyObject *, PyObject *)’                       \
  uct _object * (*)(struct pyrf_evsel *, struct _object *, struct _object *)’} to     \
  ‘PyObject * (*)(PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _objeuct \
  _object *)’} [-Werror=cast-function-type]
     .ml_meth  = (PyCFunction)pyrf_evsel__open,

The problem with the PyMethodDef::ml_meth callback is that its type is
determined based on the PyMethodDef::ml_flags value, which we set as
METH_VARARGS | METH_KEYWORDS.

That indicates that the callback is expecting an extra PyObject* arg, and is
actually PyCFunctionWithKeywords type, but the base PyMethodDef::ml_meth type
stays PyCFunction.

Previous gccs did not find this, gcc8 now does. Fixing this by silencing this
warning for python.c build.

Commiter notes:

Do not do that for CC=clang, as it breaks the build in some clang
versions, like the ones in fedora up to fedora27:

  fedora:25:error: unknown warning option '-Wno-cast-function-type'; did you mean '-Wno-bad-function-cast'? [-Werror,-Wunknown-warning-option]
  fedora:26:error: unknown warning option '-Wno-cast-function-type'; did you mean '-Wno-bad-function-cast'? [-Werror,-Wunknown-warning-option]
  fedora:27:error: unknown warning option '-Wno-cast-function-type'; did you mean '-Wno-bad-function-cast'? [-Werror,-Wunknown-warning-option]
  #

those have:

  clang version 3.9.1 (tags/RELEASE_391/final)

The one in rawhide accepts that:

  clang version 6.0.0 (tags/RELEASE_600/final)

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Link: http://lkml.kernel.org/r/20180319082902.4518-2-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>