runtime-lite has not been added as maven module

`runtime-lite` has not been added as a maven module and so it won't build with the rest of the project.

It needs an entry in the `` section of `runtime/pom.xml`, but when added it gives some compile errors

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.11.0:testCompile (default-testCompile) on project runtime-lite: Compilation failure: Compilation failure:
[ERROR] appengine-java-standard/runtime/lite/src/test/java/com/google/appengine/runtime/lite/RequestManagerTest.java:[295,63] cannot find symbol
[ERROR] symbol: method getGeneratedRegistry()
[ERROR] location: class com.google.protobuf.ExtensionRegistry
[ERROR] appengine-java-standard/runtime/lite/src/test/java/com/google/appengine/runtime/lite/RequestManagerTest.java:[334,63] cannot find symbol
[ERROR] symbol: method getGeneratedRegistry()
[ERROR] location: class com.google.protobuf.ExtensionRegistry
[ERROR] -> [Help 1]
```