Add a newly-required Error Prone option. (#2978)

Author: eamonnmcmanus

pom.xml

@@ -219,6 +219,7 @@
             <compilerArgs>
               <!-- Args related to Error Prone, see: https://errorprone.info/docs/installation#maven -->
               <arg>-XDcompilePolicy=simple</arg>
+              <arg>-XDaddTypeAnnotationsToSymbol=true</arg>
               <arg>--should-stop=ifError=FLOW</arg>
               <arg>-Xplugin:ErrorProne
                 -XepExcludedPaths:.*/generated-test-sources/protobuf/.*