Ignore Proguard warnings for Kotlin meta-annotations

Allows Java clients to use Kotlin-based annotations that aren't
dependent on the Kotlin standard library.

Bug: 146897843
Test: tested in google3
Change-Id: I0a045414f118c48f642da48e55ad3aaf0c3372e1
diff --git a/annotation/annotation-experimental/build.gradle b/annotation/annotation-experimental/build.gradle
index 7d83b43..920f1791 100644
--- a/annotation/annotation-experimental/build.gradle
+++ b/annotation/annotation-experimental/build.gradle
@@ -37,6 +37,10 @@
         sourceCompatibility JavaVersion.VERSION_1_7
         targetCompatibility JavaVersion.VERSION_1_7
     }
+
+    buildTypes.all {
+        consumerProguardFiles("proguard-rules.pro")
+    }
 }
 
 androidx {