liblightdm-gobject: define common-user GIR type

[ousamabenyounes]

Summary

• annotate the internal common-user property with its concrete introspection type
• reject generated GIR files that contain unnamed <type> elements when xmllint is available
• detect xmllint at configure time and distinguish inspection errors from malformed type metadata

Test verification (RED → GREEN)

• RED check (same check with the production annotation removed): 1/1 new checks failed:

  $ make check DEBUG=true
  LightDM-1.gir contains an unnamed type
  make[2]: *** [Makefile:1039: check-local] Error 1
  

• GREEN check (same check with the annotation restored): 1/1 new checks passed:

  $ make check DEBUG=true
  Testsuite summary for lightdm 1.33.1
  # TOTAL: 352
  # PASS:  352
  # FAIL:  0
  # ERROR: 0
  

• pinned downstream parser reproduction: gtk-rs gir at 2ae5101 reports Missing name attribute on <type> element before the fix and no error after it

• full local suite, Ubuntu rolling: 352/352 passed

• full local suite, Fedora latest: 352/352 passed

Fixes #463

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