Restore access to legacy logging flags

cc3d0a52ec70c702819c6a44f4b59eb5db3d8e37 broke access to legacy
logging flags. Earlier anybody who inherited c.g.gwt.core.Core
would be able to use logging flags. After that commit it started
requiring to inherit one of c.g.gwt.logging.*. I thought that
was already the case in GWT 2.7 but I was incorrect.

This patch restores back original behavior.

Change-Id: I55df899403dcce1093765d20625b9e91e3d4f083
diff --git a/user/src/com/google/gwt/core/Core.gwt.xml b/user/src/com/google/gwt/core/Core.gwt.xml
index 3d4ffbf..67ab57f 100644
--- a/user/src/com/google/gwt/core/Core.gwt.xml
+++ b/user/src/com/google/gwt/core/Core.gwt.xml
@@ -83,4 +83,6 @@
   <define-property name="jre.logging.simpleConsoleHandler" values="ENABLED, DISABLED" />
   <set-property name="jre.logging.simpleConsoleHandler" value="ENABLED"/>
 
+  <!-- To support legacy logging flags -->
+  <inherits name="com.google.gwt.logging.LogImpl"/>
 </module>