www/osrm-backend: fix build post LLVM 19 import

After the base system compiler was updated to Clang 19, the port failed
to build. (The specific bug is in its version of sol2 included in this
port). To fix this, I've added a patch that uses an older version of
Clang on FreeBSD-CURRENT after the update to clang 19 was applied.
It's a bit heavy-handed, but at least it gets the port building again on
CURRENT until I can find some time to make a more permanent fix for the
library.

PR: 282641