Add missing export declaration to the manifest of vaadin-push.jar (#18190)

Change-Id: I2874d3a065ad825e40e71c0167fc9ffb48ee1f5d

push/build.xml

@@ -87,6 +87,7 @@
 
     <target name="jar" depends="vaadinPush.js">
         <antcall target="common.jar">
+            <param name="osgi.extra.package.prefixes" value="VAADIN" />
             <param name="require-bundle"
                 value="com.vaadin.external.atmosphere.runtime;bundle-version=&quot;${atmosphere.runtime.version}&quot;;visibility:=reexport" />
             <reference torefid="extra.jar.includes" refid="jar.includes" />