devel/ignition-msgs: fix build on armv7
On armv7, some hashing code requires CityHash. Add libasl_city.so to
the link to get the required symbols.
This port is really out of date and should either by updated or
mercy-killed at some point.
Approved by: portmgr (build fix blanket)
MFH: 2025Q3
(cherry picked from commit 165e11b9dd5ddcbfc18fa8ff5b7c204836ae600b)