emulators/fbsd-duckstation: Fix build on 16-CURRENT

Shipped fmt does not build with clang 21 and consteval enabled:

/wrkdirs/usr/ports/emulators/fbsd-duckstation/work/duckstation-0.1-6937/dep/fmt/src/os.cc:172:35: error: call to consteval function 'fmt::basic_format_string<char, const char *>::basic_format_string<FMT_COMPILE_STRING, 0>' is not a constant expression

172 |     FMT_THROW(system_error(errno, FMT_STRING("cannot open file {}"),
    |                                   ^

/wrkdirs/usr/ports/emulators/fbsd-duckstation/work/duckstation-0.1-6937/dep/fmt/include/fmt/format.h:1905:23: note: expanded from macro 'FMT_STRING'
1905 | #define FMT_STRING(s) FMT_STRING_IMPL(s, fmt::detail::compile_string, )

|                       ^

[...]

so disable consteval to work-around the problem.