rpc: fix compilation error caused by fmt::runtime()

[pwrobelse]

Depending on the 'SEASTAR_LOGGER_COMPILE_TIME_FMT' the declaration of 'logger::log()'
function may use either 'fmt::format_string' or 'const char*' format parameter.

In the case of the usage of 'const char*', there is a need to call 'fmt::runtime()' function to
obtain format_string.

When the project was built on Ubuntu 22.04 using the steps described in the tutorial,
there was a compilation error related to passing argument with type 'fmt::format_string<>'
to 'fmt::runtime()' that handles characters strings.

This change introduces contexpr if statement to remove the compilation error and use
'fmt::runtime()' only when the function takes 'const char*'.

[pwrobelse]

The error message that was seen on Ubuntu 22.04:

/home/ubuntu/repo/seastar/include/seastar/rpc/rpc.hh:221:60: error: no matching function for call to ‘runtime(fmt::v8::format_string<const seastar::socket_address&, long int&, std::basic_string_view<char, std::char_traits<char> >&>&)’
  221 |             fmt::format_to(fmt::appender(out), fmt::runtime(fmt), std::forward<Args>(args)...);

[pwrobelse]

Perhaps, the problem started to be seen after merging: e3249a8

[tchaikov]

probably a better fix is to conditionalize this part with SEASTAR_LOGGER_COMPILE_TIME_FMT as well.

[tchaikov]

probably a better fix is to conditionalize this part with SEASTAR_LOGGER_COMPILE_TIME_FMT as well.

created #2425