Add slf4j_simple to the top level Gradle build

build.gradle

@@ -140,6 +140,7 @@ ext.library = [
     proto_google_cloud_spanner_admin_database_v1: "com.google.api.grpc:proto-google-cloud-spanner-admin-database-v1:$grpc_google_common_protos",
     proto_google_common_protos: "com.google.api.grpc:proto-google-common-protos:$grpc_google_common_protos",
     slf4j_api: "org.slf4j:slf4j-api:1.7.25",
+    slf4j_simple: "org.slf4j:slf4j-simple:1.7.25",
     slf4j_jdk14: "org.slf4j:slf4j-jdk14:1.7.25",
     slf4j_log4j12: "org.slf4j:slf4j-log4j12:1.7.25",
     snappy_java: "org.xerial.snappy:snappy-java:1.1.4",