Add missing namespace and XSD to SpotBugs filter file.

Author: Gary Gregory

src/conf/spotbugs-exclude-filter.xml

@@ -15,7 +15,10 @@
    See the License for the specific language governing permissions and
    limitations under the License.
 -->
-<FindBugsFilter>
+<FindBugsFilter
+    xmlns="https://github.com/spotbugs/filter/3.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="https://github.com/spotbugs/filter/3.0.0 https://raw.githubusercontent.com/spotbugs/spotbugs/3.1.0/spotbugs/etc/findbugsfilter.xsd">
 
   <!--  See discussion on https://issues.apache.org/jira/browse/IO-216 -->
   <Match>