Fix -Wnon-virtual-dtor errors

Summary: This breaks mongo+rocks build https://mci.10gen.com/task_log_raw/rocksdb_ubuntu1404_rocksdb_c6e8e3d868660dc66b3bbd438cdc135df6356c5a_14_11_10_21_36_10_compile_ubuntu1404_rocksdb/0?type=T

Test Plan: m check + -Wnon-virtual-dtor

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28653

Makefile

@@ -69,7 +69,7 @@ install:
 	@[ ! -e $(SHARED) ] || install -C -m 644 $(SHARED) $(INSTALL_PATH)/lib
 #-------------------------------------------------
 
-WARNING_FLAGS = -Wall -Werror -Wsign-compare -Wshadow
+WARNING_FLAGS = -Wall -Werror -Wsign-compare -Wshadow -Wnon-virtual-dtor
 CFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CCFLAGS) $(OPT)
 CXXFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CXXFLAGS) $(OPT) -Woverloaded-virtual
 

db/wal_manager.h

@@ -38,9 +38,9 @@ class WalManager {
         env_(db_options.env),
         purge_wal_files_last_run_(0) {}
 
-  virtual Status GetSortedWalFiles(VectorLogPtr& files);
+  Status GetSortedWalFiles(VectorLogPtr& files);
 
-  virtual Status GetUpdatesSince(
+  Status GetUpdatesSince(
       SequenceNumber seq_number, std::unique_ptr<TransactionLogIterator>* iter,
       const TransactionLogIterator::ReadOptions& read_options,
       VersionSet* version_set);

include/rocksdb/listener.h

@@ -58,6 +58,7 @@ class EventListener {
       const std::string& file_path,
       bool triggered_writes_slowdown,
       bool triggered_writes_stop) {}
+  virtual ~EventListener() {}
 };
 
 }  // namespace rocksdb

util/arena.h

@@ -69,7 +69,7 @@ class Arena {
 
   // If an allocation is too big, we'll allocate an irregular block with the
   // same size of that allocation.
-  virtual size_t IrregularBlockNum() const { return irregular_block_num; }
+  size_t IrregularBlockNum() const { return irregular_block_num; }
 
   size_t BlockSize() const { return kBlockSize; }