jgvariant-tool: Mark compile-time-only dependencies as optional.

The way flatten-maven-plugin is configured, optional dependencies are
omitted, but provided dependencies are included in the final POM.
Since they are not needed at runtime, it makes sense to omit them by
marking them as optional.  This causes fewer security alerts.

Change-Id: I8f39c0066331c067186b9fa016bf60477653a48c
diff --git a/jgvariant-tool/pom.xml b/jgvariant-tool/pom.xml
index ac21749..f4026e4 100644
--- a/jgvariant-tool/pom.xml
+++ b/jgvariant-tool/pom.xml
@@ -50,16 +50,19 @@
       <groupId>com.google.errorprone</groupId>
       <artifactId>error_prone_annotations</artifactId>
       <scope>provided</scope>
+      <optional>true</optional>
     </dependency>
     <dependency>
       <groupId>org.jetbrains</groupId>
       <artifactId>annotations</artifactId>
       <scope>provided</scope>
+      <optional>true</optional>
     </dependency>
     <dependency>
       <groupId>org.apiguardian</groupId>
       <artifactId>apiguardian-api</artifactId>
       <scope>provided</scope>
+      <optional>true</optional>
     </dependency>
 
     <!-- Command line tooling -->