Fixed a compile warning in rocksjni/loggerjnicallback.cc

Summary:
This patch fixes the following compile warning.
    java/rocksjni/loggerjnicallback.cc: In constructor ‘rocksdb::LoggerJniCallback::LoggerJniCallback(JNIEnv*, jobject)’:
    java/rocksjni/loggerjnicallback.cc:19:14: warning: unused variable ‘rs’ [-Wunused-variable]
       const jint rs = env->GetJavaVM(&m_jvm);
                  ^

Test Plan: make rocksdbjavastaticrelease

Reviewers: sdong, anthony, IslamAbdelRahman, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D45981

java/rocksjni/loggerjnicallback.cc

@@ -15,8 +15,7 @@ namespace rocksdb {
 
 LoggerJniCallback::LoggerJniCallback(
     JNIEnv* env, jobject jlogger) {
-
-  const jint rs = env->GetJavaVM(&m_jvm);
+  const jint rs __attribute__((unused)) = env->GetJavaVM(&m_jvm);
   assert(rs == JNI_OK);
 
   // Note: we want to access the Java Logger instance