Silence GCC8's -Wcast-function-type. (#1396)

* Silence GCC8's -Wcast-function-type.

See https://bugs.python.org/issue33012 and PRs linked therein.

Author: anntzer

include/pybind11/pybind11.h

@@ -306,7 +306,7 @@ class cpp_function : public function {
             rec->def = new PyMethodDef();
             std::memset(rec->def, 0, sizeof(PyMethodDef));
             rec->def->ml_name = rec->name;
-            rec->def->ml_meth = reinterpret_cast<PyCFunction>(*dispatcher);
+            rec->def->ml_meth = reinterpret_cast<PyCFunction>(reinterpret_cast<void (*) (void)>(*dispatcher));
             rec->def->ml_flags = METH_VARARGS | METH_KEYWORDS;
 
             capsule rec_capsule(rec, [](void *ptr) {