Add missing declaration of initperl to avoid an implicit function
declaration in Python.xs.

Submitted upstream: <https://github.com/niner/inline-python-pm/pull/39>

diff --git a/perlmodule.h b/perlmodule.h
index 1d336f4fe436a1d7..8ec36c324a703387 100644
--- a/perlmodule.h
+++ b/perlmodule.h
@@ -76,6 +76,8 @@ extern PyObject * newPerlSub_object(PyObject *, PyObject *, SV *);
 extern PyObject * newPerlMethod_object(PyObject*, PyObject*, SV*);
 extern PyObject * newPerlCfun_object(PyObject* (*)(PyObject *, PyObject *));
 
+extern void initperl(void);
+
 #ifdef __cplusplus
 }
 #endif