sortstring: fix compiler warning
sortstring: fix compiler warning

Get rid of compiler warning:

  sortstring.cpp: In function ‘void system(const std::string&)’:
  sortstring.cpp:49: warning: ignoring return value of ‘int system(const char*)’,
     declared with attribute warn_unused_result     



rantala/string-sorting,rantala/string-sorting,rantala/string-sorting,rantala/string-sorting