Doc: Excluded Dialogs directory from Qt Quick doc build.

Qt Quick Dialogs is separate and should be excluded from the Qt Quick
doc build.

Task-number: QTBUG-30180
Change-Id: I6e63efc071d8d2e5673745f0dcd97d5b5f562d25
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>

src/quick/doc/qtquick.qdocconf

@@ -58,3 +58,6 @@ sourcedirs += ../../imports
 #add plugins directory because of dependencies
 headerdirs += ../../plugins
 sourcedirs += ../../plugins
+
+#exclude certain directories
+excludedirs = ../../imports/dialogs