libxmlb: upgrade 0.3.18 -> 0.3.19

Changelog:
============
- Add xb_version_string() to get the runtime ABI version
- Add the runtime version as the default XMLb invalidation GUID

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>