Use API Guardian annotations.

Change-Id: Icfd30e9b06e5c7ee9f51c6879bfe4978a6f44600
diff --git a/src/main/java/module-info.java b/src/main/java/module-info.java
index 39e91b8..c0debad 100644
--- a/src/main/java/module-info.java
+++ b/src/main/java/module-info.java
@@ -8,6 +8,7 @@
 module eu.mulk.jgvariant.core {
   requires com.google.errorprone.annotations;
   requires org.jetbrains.annotations;
+  requires org.apiguardian.api;
 
   exports eu.mulk.jgvariant.core;
 }