[fixes #3310] Ignore non-static/non-public extension methods

test/transform/resource/after-ecj/ExtensionMethodNonStatic.java

@@ -0,0 +1,18 @@
+import lombok.experimental.ExtensionMethod;
+@ExtensionMethod({ExtensionMethodNonStatic.Extensions.class}) class ExtensionMethodNonStatic {
+  static class Extensions {
+    Extensions() {
+      super();
+    }
+    public boolean startsWith(String s, String prefix) {
+      return s.startsWith(prefix);
+    }
+  }
+  ExtensionMethodNonStatic() {
+    super();
+  }
+  public void test() {
+    String s = "test";
+    s.startsWith("");
+  }
+}