oc: Use enum mapistore_error instead of int

This specifies a little the scope of the variable to make it
more realistic with the actual values it may have. We do have
a static typed compiled language, why don't we use it?

OpenChange/MAPIStoreObjectProxy.m

@@ -51,7 +51,7 @@
   MAPIStorePropertyGetter method;
   uint16_t propValue;
   SEL methodSel;
-  int rc;
+  enum mapistore_error rc;
 
   propValue = (propTag & 0xffff0000) >> 16;
   methodSel = MAPIStoreSelectorForPropertyGetter (propValue);