Fix generics compiler warning

Author: garydgregory

src/main/java/org/apache/commons/io/input/ObservableInputStream.java

@@ -165,7 +165,7 @@ public void finished() throws IOException {
 
     private final List<Observer> observers;
 
-    ObservableInputStream(final AbstractBuilder builder) throws IOException {
+    ObservableInputStream(final AbstractBuilder<?> builder) throws IOException {
         super(builder);
         this.observers = builder.observers;
     }