Make SynthetizedAnnotation public

Enable public visibility on SynthetizedAnnotation to allow annotation
outside its package to be proxied properly. This commit is pending a
unit test that actually reproduces the problem.

Issue: SPR-13057

Author: snicoll

spring-core/src/main/java/org/springframework/core/annotation/SynthesizedAnnotation.java

@@ -24,5 +24,5 @@
  * @author Sam Brannen
  * @since 4.2
  */
-interface SynthesizedAnnotation {
+public interface SynthesizedAnnotation {
 }