fdisk.c: add partition module object

Call Partition_AddModuleObject when initializing the python module.

Fixes bug when using the Partition class but the class has not
been added to the module via Py_TypeReady.

A common error was the type not being ready (missing attributes):

>>> for pa in cxt.parts:
...     print(pa.partno)
...
Traceback (most recent call last):
   File "<stdin>", line 2, in <module>
AttributeError: 'libfdisk.Partition' object has no attribute 'partno'

fdisk.c

@@ -65,6 +65,7 @@ PyInit_fdisk(void)
 
 	Context_AddModuleObject(m);
 	Label_AddModuleObject(m);
+	Partition_AddModuleObject(m);
 
 
 	return m;

fdisk.h

@@ -44,6 +44,7 @@ extern PyTypeObject PartitionType;
 
 extern void Context_AddModuleObject(PyObject *mod);
 extern void Label_AddModuleObject(PyObject *mod);
+extern void Partition_AddModuleObject(PyObject *mod);
 
 extern PyObject *PyObjectResultStr(const char *s);
 extern PyObject *PyObjectResultLabel(struct fdisk_label *lb);