libgsf: update to version 1.14.26

Previous version was 1.14.21, but Gnumeric requires at least 1.14.24.
Since we are changing it, we pick the latest available version.

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>