install_prereq: Update FreeBSD libraries.

Because the code review system Gerrit creates merge conflicts even when one line
apart another change happened, the previous update to the FreeBSD libraries had
to be rebased via Git. Because of a break for training of the original
contributor, this rebase was done by another contributor and the variant for
Asterisk 13 was cherry-picked to all branches. By this, dependencies for new
features added in newer Asterisk version got lost. This can be seen, when not
the original path set but a previous patch set is compared.

This change here fixes this by adding those (optional) dependencies for
Asterisk 15 and newer (again).

ASTERISK-27686

Change-Id: I6638a3d0dc37ad4ff5f94be15463e3dd8a2bfe74