Use PyMODINIT_FUNC for module init functions

This is the correct way to define init functions
and allows greenlet to pick up any platform
specific external linkage attributes, including
extern "C" when compiled as C++ code.

greenlet.c

@@ -1265,12 +1265,13 @@ static struct PyModuleDef greenlet_module_def = {
 	GreenMethods,
 };
 
-PyObject *
+PyMODINIT_FUNC
 PyInit_greenlet(void)
 #else
 #define INITERROR return
 
-void initgreenlet(void)
+PyMODINIT_FUNC
+initgreenlet(void)
 #endif
 {
 	PyObject* m = NULL;

tests/_test_extension.c

@@ -168,7 +168,7 @@ static struct PyModuleDef moduledef = {
 	NULL
 };
 
-PyObject *
+PyMODINIT_FUNC
 PyInit__test_extension(void)
 #else
 #define INITERROR return