Add QIcon::fallbackThemeName() as fallback

This seems to work on NeoChat when using Awaita as icon theme with
Breeze as fallback.

src/kiconloader.cpp

@@ -684,6 +684,7 @@ void KIconLoaderPrivate::addBaseThemes(KIconThemeNode *node, const QString &appn
     // present in an inherited theme, and hicolor goes last.
 
     addInheritedThemes(node, appname);
+    addThemeByName(QIcon::fallbackThemeName(), appname);
     addThemeByName(QStringLiteral("hicolor"), appname);
 }