Include gradle files into the source artifact for releases, exclude site2/**

### Motivation

gradle files weren't included into the source artifact of the release
site2/ was included.

### Changes

Added gradle into the include pattern.
Added site2 into the exclude patterns.



Reviewers: Enrico Olivelli <eolivelli@gmail.com>

This closes apache#2714 from dlg99/gradle-release

Author: dlg99

bookkeeper-dist/src/assemble/src.xml

@@ -31,6 +31,7 @@
         <include>**/LICENSE</include>
         <include>**/NOTICE</include>
         <include>**/pom.xml</include>
+        <include>**/*gradle*</include>
         <include>**/src/**</include>
         <include>**/conf/**</include>
         <include>**/bin/**</include>
@@ -59,6 +60,7 @@
         <exclude>dev/**</exclude>
         <!-- site -->
         <exclude>site/**</exclude>
+        <exclude>site2/**</exclude>
         <!-- jenkins -->
         <exclude>jenkins/**</exclude>
       </excludes>