add hibernate-jpamodelgen to bom

[xfh]

To generate the static metamodel, e.g. for criteria queries, the
hibernate-jpamodelgen dependency must be added.

Adding the dependency in the dependency management section of the
bom makes it easier to keep the versions of hibernate and the
metamodel generator in sync.

Each project using the metamodel still needs to add an explicit
dependency as in

<dependencies>
   ...
    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-jpamodelgen</artifactId>
    </dependency>
</dependencies>

[gsmet]

That's indeed a good idea. Thanks for that!

[machi1990]

CI failure looks totally unrelated with the change

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.0-jboss-2:testCompile (default-testCompile) on project quarkus-integration-test-vertx-http: Compilation failure
[ERROR] /home/vsts/work/1/s/integration-tests/vertx-http/src/test/java/io/quarkus/it/vertx/VertxProducerResourceTest.java:[21,29] cannot find symbol
[ERROR]   symbol:   method ssl()
[ERROR]   location: @interface io.quarkus.test.common.http.TestHTTPResource
[ERROR] -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException
The process '/usr/share/apache-maven-3.6.1/bin/mvn' failed with exit code 1
Could not retrieve code analysis results - Maven run failed.