cogl/gir: Depend explicitly on Mtk

As it is used now for MtkRegion types making the GIR file not containing
the relevant dependency and not being able to resolve MtkRegion type

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3292>

doc/reference/cogl-pango/meson.build

@@ -14,6 +14,7 @@ custom_target('cogl-pango-doc',
     gidocgen,
     'generate',
     gidocgen_common_args,
+    '--add-include-path=@0@'.format(meson.current_build_dir() / '../../../mtk/mtk'),
     '--add-include-path=@0@'.format(meson.current_build_dir() / '../../../cogl/cogl'),
     '--config=@INPUT0@',
     '--output-dir=@OUTPUT@',