fflush(stderr) call improvement in signal_handler.cc

In https://github.com/MariaDB/server/blob/10.6/sql/signal_handler.cc, the following flush happens in one case:

#else

  char buff[80];

  my_getwd(buff, sizeof(buff), 0);

  my_safe_printf_stderr("Writing a core file at %s\n", buff);

  fflush(stderr);                                                                          <<<<<<<<<<<<<<<<<<

#endif

Yet, there are three other parts just above it that also write to stderr in similar ways, and that do not flush stderr.

Suggested fix; either add fflush(stderr); to the other three parts above it too, or move the fflush(stderr); to just outside the #endif so it may always be called irrespective (best solution imho; less code duplication and future proof)