Make GroovyBeanDefinitionSource accessible to the CLI again

It was made package-private in 31b0e817 but it needs to be protected
due to its use in the CLI's GroovyBeansTransformation.

spring-boot/src/main/java/org/springframework/boot/BeanDefinitionLoader.java

@@ -326,7 +326,8 @@ class BeanDefinitionLoader {
 	/**
 	 * Source for Bean definitions defined in Groovy.
 	 */
-	interface GroovyBeanDefinitionSource {
+	@FunctionalInterface
+	protected interface GroovyBeanDefinitionSource {
 
 		Closure<?> getBeans();