jme3-example: add 2 library dependencies that were missing

jme3-examples/build.gradle

@@ -24,6 +24,8 @@ dependencies {
     implementation project(':jme3-networking')
     implementation project(':jme3-niftygui')
     implementation project(':jme3-plugins')
+    implementation project(':jme3-plugins-json')
+    implementation project(':jme3-plugins-json-gson')
     implementation project(':jme3-terrain')
     implementation project(':jme3-awt-dialogs')
     runtimeOnly project(':jme3-testdata')