devel/spdlog: fix build on GCC architectures

assert() is not declared by default:
In file included from /wrkdirs/usr/ports/devel/spdlog/work/spdlog-1.4.2/src/async.cpp:11:
/wrkdirs/usr/ports/devel/spdlog/work/spdlog-1.4.2/include/spdlog/details/thread_pool-inl.h: In member function 'bool spdlog::details::thread_pool::process_next_msg_()':
/wrkdirs/usr/ports/devel/spdlog/work/spdlog-1.4.2/include/spdlog/details/thread_pool-inl.h:119:9: error: 'assert' was not declared in this scope

119 |         assert(false && "Unexpected async_msg_type");
    |         ^~~~~~

PR: 242825
Approved by: vanilla (maintainer)