GUACAMOLE-2258: Forgot the import needed for the previous change

Author: David Bateman

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-openid/src/main/java/org/apache/guacamole/auth/openid/token/TokenValidationService.java

@@ -20,6 +20,7 @@
 package org.apache.guacamole.auth.openid.token;
 
 import com.google.inject.Inject;
+import java.io.IOException;
 import java.net.URLEncoder;
 import java.nio.charset.StandardCharsets;
 import java.util.Collection;