Doesn't check for required itstool for gnome.yelp

[hadess]

Using meson-0.55.1-1.fc34 trying to build malcontent, which uses gnome.yelp would fail with:

Traceback (most recent call last):
  File "/usr/bin/meson", line 33, in <module>
    sys.exit(load_entry_point('meson==0.55.1', 'console_scripts', 'meson')())
  File "/usr/lib/python3.9/site-packages/mesonbuild/mesonmain.py", line 228, in main
    return run(sys.argv[1:], launcher)
  File "/usr/lib/python3.9/site-packages/mesonbuild/mesonmain.py", line 217, in run
    return run_script_command(args[1], args[2:])
  File "/usr/lib/python3.9/site-packages/mesonbuild/mesonmain.py", line 165, in run_script_command
    return module.run(script_args)
  File "/usr/lib/python3.9/site-packages/mesonbuild/scripts/yelphelper.py", line 130, in run
    merge_translations(build_subdir, abs_sources, langs)
  File "/usr/lib/python3.9/site-packages/mesonbuild/scripts/yelphelper.py", line 57, in merge_translations
    subprocess.call([
  File "/usr/lib64/python3.9/subprocess.py", line 349, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib64/python3.9/subprocess.py", line 947, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib64/python3.9/subprocess.py", line 1819, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'itstool'

if itstool is missing.