plpython: add missing plpython.h include to plpy_plpymodule.h

The include was missing before 9b7e24a2cb, but starting with that commit the
missing include causes cpluspluscheck to fail because the use of
PyMODINIT_FUNC isn't incidentally protected by an ifdef anymore.

Discussion: https://postgr.es/m/20220308045916.7baapelbgftoqeop@alap3.anarazel.de

src/pl/plpython/plpy_plpymodule.h

@@ -5,6 +5,7 @@
 #ifndef PLPY_PLPYMODULE_H
 #define PLPY_PLPYMODULE_H
 
+#include "plpython.h"
 #include "utils/hsearch.h"
 
 /* A hash table mapping sqlstates to exceptions, for speedy lookup */