build: Increase libassuan min version to 2.5.0

assuan_sock_set_system_hooks is used unconditionally in gnupg since
commit 9f641430dcdecbd7ee205d407cb19bb4262aa95d, and as such it requires
libassuan 2.5.0 (function introduced in
commit 90dc81682b13a7cf716a8a26b891051cbd4b0caf)

For a detailed description see:
https://lists.gnupg.org/pipermail/gnupg-devel/2017-December/033323.html