build: Prepare release with API change.
* configure.ac (LIBASSUAN_CONFIG_API_VERSION): Increment. (LIBASSUAN_LT_CURRENT): Increment for adding new interface. (LIBASSUAN_LT_AGE): Likewise. (LIBASSUAN_LT_REVISION): Reset to 0.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>