Fix conversion warning on FreeBSD #2140
Description
When enabling -Wconversion on our FreeBSD 14 build, it fails:
In file included from /tmp/cirrus-ci-build/libcaf_core/caf/abstract_actor.cpp:11:
In file included from /tmp/cirrus-ci-build/libcaf_core/caf/actor_system.hpp:12:
In file included from /tmp/cirrus-ci-build/libcaf_core/caf/detail/format.hpp:18:
/tmp/cirrus-ci-build/libcaf_core/caf/detail/source_location.hpp:37:17: error: implicit conversion changes signedness: 'int' to 'uint_least32_t' (aka 'unsigned int') [-Werror,-Wsign-conversion]
37 | loc.line_ = line;
| ~ ^~~~
FreeBSD is using Clang 18.1.6, so it might not FreeBSD-specific.