Change PyInit_plpy to external linkage

Module initialization functions in Python 3 must have external
linkage, because PyMODINIT_FUNC does dllexport on Windows-like
platforms.  Without this change, the build with Python 3 fails on
Windows.

src/pl/plpython/plpython.c

@@ -4066,7 +4066,13 @@ PLy_add_exceptions(PyObject *plpy)
 }
 
 #if PY_MAJOR_VERSION >= 3
-static PyMODINIT_FUNC
+/*
+ * Must have external linkage, because PyMODINIT_FUNC does dllexport on
+ * Windows-like platforms.
+ */
+PyMODINIT_FUNC PyInit_plpy(void);
+
+PyMODINIT_FUNC
 PyInit_plpy(void)
 {
 	PyObject   *m;