you need to add APLN to boot classpath.
see [wildfly-dev] ALPN support for more details on installation
in short you need to grab alpn-boot jar Maven Repository: org.mortbay.jetty.alpn » alpn-boot that is appropriate for exact version of JDK.
see the mapping here http://eclipse.org/jetty/documentation/current/alpn-chapter.html#alpn-versions
and than make sure it is on boot classpath, either by adding the jar to endorsed dir of JDK or by modifying start up parameters of wildfly to include -Xbootclasspath/p:<path-to-your-jar>
Thank you, Tomaz, for your prompt response! It works fine.
Does it mean that there is no way to enable HTTP/2 unless your are running OpenJDK?