rsgain.cpp: use C++ cmath

[barracuda156]

Current code uses C99 header and isfinite in C++ file, which fails to compile on some configs. Fix that.

[barracuda156]

/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_audio_rsgain/rsgain/work/rsgain-3.5.3/src/rsgain.cpp: In function 'bool parse_max_peak_level(const char*, double&)':
/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_audio_rsgain/rsgain/work/rsgain-3.5.3/src/rsgain.cpp:153:27: error: 'isfinite' was not declared in this scope; did you mean 'std::isfinite'?
  153 |     if (rest == value || !isfinite(max_peak)) {
      |                           ^~~~~~~~
      |                           std::isfinite
In file included from /opt/local/include/LegacySupport/cmath:54,
                 from /opt/local/include/libfmt9/fmt/chrono.h:13,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_audio_rsgain/rsgain/work/rsgain-3.5.3/src/output.hpp:66,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_audio_rsgain/rsgain/work/rsgain-3.5.3/src/rsgain.cpp:58:
/opt/local/include/gcc14/c++/cmath:1142:5: note: 'std::isfinite' declared here
 1142 |     isfinite(_Tp)
      |     ^~~~~~~~
make[2]: *** [src/CMakeFiles/rsgain.dir/rsgain.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....