message: fs: add missing <cassert> include
  This is needed to prevent compilation failures once boost is removed,
  however is still correct to include now, and reduces the diff in
  <string> is extracted from the defines because it is used for Windows
  and non-Windows code, i.e get_filesystem_error_message().
diff: #ifndef WIN32
  #include <cstring>
  #include <fcntl.h>
  -#include <string>
  #include <sys/file.h>
  #include <sys/utsname.h>
  #include <unistd.h>
  #include <windows.h>
  #endif
  +#include <cassert>
  +#include <string>
  +
  namespace fsbridge {
  FILE *fopen(const fs::path& p, const char *mode)
  