gcompris: fix loading of image assets

Previously some games were not able to load some WebP image assets,
eg. the Erase game failed to display any cute background image,
making it unattractive:

> qrc:/gcompris/src/activities/erase/Erase.qml:26:20: QML Image:
> Error decoding: qrc:/gcompris/src/activities/erase/resource/butterfly.webp:
> Unsupported image format

pkgs/games/gcompris/default.nix

@@ -11,6 +11,7 @@
 , qtcharts
 , qtdeclarative
 , qtgraphicaleffects
+, qtimageformats
 , qtmultimedia
 , qtquickcontrols2
 , qtsensors
@@ -41,6 +42,7 @@ stdenv.mkDerivation (finalAttrs: {
     qtcharts
     qtdeclarative
     qtgraphicaleffects
+    qtimageformats
     qtmultimedia
     qtquickcontrols2
     qtsensors