Log Message:
Update libsieve to 2.2.7.

Changes:
libSieve 2.2.7
--------------

- Fixed a duplicate symbol definition (thanks to Wilfried Goesgens).

- Fixed a mismatched function declaration (thanks to Eugene Prokopiev).

- More cases where Vacation should be ignored (thanks to Dilyan Palauzov).

- Generate Vacation handle according to RFC 5230 (thanks to Dilyan Palauzov).

- Use of NULL without declaration (thanks to Dilyan Palauzov).

- Added pkgconfig support (thanks to Dilyan Palauzov).

- Documentation corrections (thanks to Dilyan Palauzov).

- Use left-recursion when parsing stringlists (thanks to Dilyan Palauzov).