TOOLS: Fix warning Wsign-compare
    
    src/tools/tools_util.c: In function ‘parse_groups’:
    src/tools/tools_util.c:116:19: error: comparison between
        signed and unsigned integer expressions [-Werror=sign-compare]
         for (i = 0; i < tokens; i++) {
                       ^
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>