deps/spdlog: fix conversion from const string to const char*

[illwieckz]

Fixes:

In file included from LuxCore/deps/spdlog-1.8.0/include/spdlog/common.h:245,
                 from LuxCore/deps/spdlog-1.8.0/include/spdlog/spdlog.h:12,
                 from LuxCore/src/luxcore/luxcore.cpp:22:
LuxCore/deps/spdlog-1.8.0/include/spdlog/common-inl.h: In constructor ‘spdlog::spdlog_ex::spdlog_ex(const string&, int)’:
LuxCore/deps/spdlog-1.8.0/include/spdlog/common-inl.h:57:50: error: cannot convert ‘const string’ {aka ‘const std::__cxx11::basic_string<char>’} to ‘const char*’
   57 |     fmt::format_system_error(outbuf, last_errno, msg);
      |                                                  ^~~
      |                                                  |
      |                                                  const string {aka const std::__cxx11::basic_string<char>}

It looks like this is fixed upstream so one may prefer to update the deps instead, but this patch fixes the bug with less effort until someone looks at updating deps.

This work is courtesy of I ♥ Compute initiative funded by 🇫🇷️ rebatir.fr.