Fix Gemfire ClientConfig BeanFactoryPostProcessor

@Bean method ClientConfig.propertySourcesPlaceholderConfigurer was
non-static and returns an object assignable to Spring's
BeanFactoryPostProcessor interface. This will result in a failure to
process annotations such as @Autowired, @Resource and @PostConstruct
within the method's declaring @Configuration class.

This commit makes the method static to avoid the problem.

samples/httpsession-gemfire-clientserver/src/main/java/sample/ClientConfig.java

@@ -68,7 +68,7 @@ public class ClientConfig {
 	}
 
 	@Bean
-	PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer() {
+	static PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer() {
 		return new PropertySourcesPlaceholderConfigurer();
 	}