Fixed compile warning in rocksdbjava

Summary:
Fixed the following compile warning in rocksdbjava:

    java/rocksjni/comparatorjnicallback.cc:20:14: warning: unused variable ‘rs’ [-Wunused-variable]
       const jint rs = env->GetJavaVM(&m_jvm);
                  ^
    java/rocksjni/comparatorjnicallback.cc: In member function ‘JNIEnv* rocksdb::BaseComparatorJniCallback::getJniEnv() const’:
    java/rocksjni/comparatorjnicallback.cc:45:8: warning: unused variable ‘rs’ [-Wunused-variable]
       jint rs = m_jvm->AttachCurrentThread(reinterpret_cast<void **>(&env), NULL);
            ^
    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);
                  ^
    java/rocksjni/loggerjnicallback.cc: In member function ‘JNIEnv* rocksdb::LoggerJniCallback::getJniEnv() const’:
    java/rocksjni/loggerjnicallback.cc:33:8: warning: unused variable ‘rs’ [-Wunused-variable]
       jint rs = m_jvm->AttachCurrentThread(reinterpret_cast<void **>(&env), NULL);
            ^

Test Plan: make rocksdbjava -j32

Reviewers: sdong, anthony, IslamAbdelRahman, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D45891

java/rocksjni/comparatorjnicallback.cc

@@ -17,7 +17,7 @@ BaseComparatorJniCallback::BaseComparatorJniCallback(
     mtx_findShortestSeparator(new port::Mutex(copt->use_adaptive_mutex)) {
   // Note: Comparator methods may be accessed by multiple threads,
   // so we ref the jvm not the env
-  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 Comparator instance
@@ -42,7 +42,8 @@ BaseComparatorJniCallback::BaseComparatorJniCallback(
  */
 JNIEnv* BaseComparatorJniCallback::getJniEnv() const {
   JNIEnv *env;
-  jint rs = m_jvm->AttachCurrentThread(reinterpret_cast<void **>(&env), NULL);
+  jint rs __attribute__((unused)) =
+      m_jvm->AttachCurrentThread(reinterpret_cast<void**>(&env), NULL);
   assert(rs == JNI_OK);
   return env;
 }

java/rocksjni/loggerjnicallback.cc

@@ -30,7 +30,8 @@ LoggerJniCallback::LoggerJniCallback(
  */
 JNIEnv* LoggerJniCallback::getJniEnv() const {
   JNIEnv *env;
-  jint rs = m_jvm->AttachCurrentThread(reinterpret_cast<void **>(&env), NULL);
+  jint rs __attribute__((unused)) =
+      m_jvm->AttachCurrentThread(reinterpret_cast<void**>(&env), NULL);
   assert(rs == JNI_OK);
   return env;
 }