Make Optional.Interface repeatable (#4138)

src/main/java/net/minecraftforge/fml/common/Optional.java

@@ -20,6 +20,7 @@
 package net.minecraftforge.fml.common;
 
 import java.lang.annotation.ElementType;
+import java.lang.annotation.Repeatable;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
@@ -56,6 +57,7 @@ public final class Optional {
      * @author cpw
      *
      */
+    @Repeatable(InterfaceList.class)
     @Retention(RetentionPolicy.RUNTIME)
     @Target(ElementType.TYPE)
     public @interface Interface {