Add defaultbranch to .gitreview.

Although the default value of defaultbranch is master I'm adding it
anyways.  This way when new major branches are being created the value
can be updated instead of having to remember the name of the key.

Change-Id: I3db009217c5ae399fb84bee95076f4dbb7fa52d2

.gitreview

@@ -2,3 +2,4 @@
 host=gerrit.asterisk.org
 port=29418
 project=asterisk.git
+defaultbranch=master