chore: fix dependency error

Author: minherz

src/test/java/com/google/cloud/logging/logback/LoggingAppenderTest.java

@@ -27,7 +27,7 @@
 import ch.qos.logback.classic.filter.ThresholdFilter;
 import ch.qos.logback.classic.spi.ILoggingEvent;
 import ch.qos.logback.classic.spi.LoggingEvent;
-import com.google.api.client.util.Strings;
+import com.google.common.base.Strings;
 import com.google.cloud.MonitoredResource;
 import com.google.cloud.Timestamp;
 import com.google.cloud.logging.LogEntry;