Added support to verify reverse dependencies in a transaction.

See https://plus.google.com/106812073015308749022/posts/ZXMHimd8iKd

include/xbps_api_impl.h

@@ -176,6 +176,12 @@ xbps_dictionary_t HIDDEN xbps_find_pkg_in_dict(xbps_dictionary_t, const char *);
 xbps_dictionary_t HIDDEN xbps_find_virtualpkg_in_dict(struct xbps_handle *,
 					xbps_dictionary_t,
 					const char *);
+/**
+ * @private
+ * From lib/transaction_revdeps.c
+ */
+void HIDDEN xbps_transaction_revdeps(struct xbps_handle *);
+
 /**
  * @private
  * From lib/transaction_sortdeps.c