build-sys: add -Wno-cast-function-type for python

build-sys: add -Wno-cast-function-type for python

 libmount/python/pylibmount.c:158:19: warning: cast between incompatible function types
   from ‘PyObject * (*)(PyObject *)’ {aka ‘struct _object * (*)(struct _object *)’}
     to ‘PyObject * (*)(PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _object *, struct _object *)’}
   [-Wcast-function-type]

This is generic problem in all Python C code and gcc v8. The another
possible (and probably more correct) way is to add unused argument to
all API functions. Unfortunately, this solution is pretty invasive. The
question is if gcc is not too paranoid in this case.

For more details see https://bugs.python.org/issue33012. It seems
Python guys also prefer CFLAGS modification for now.

Signed-off-by: Karel Zak <kzak@redhat.com>