FBX: include main folder for shared plugins API

jme3-plugins/build.gradle

@@ -5,6 +5,7 @@ if (!hasProperty('mainClass')) {
 sourceSets {
     main {
         java {
+            srcDir 'src/main/java'
             srcDir 'src/ogre/java'
             srcDir 'src/fbx/java'
             srcDir 'src/xml/java'