rocksdb: Fix uninitialized use error

Summary:
When using latest clang (3.6 or 3.7/trunck) rocksdb is failing with many errors. Some errors are uninitialized use errors.

```
...
  CC       db/log_test.o
util/ldb_cmd.cc:394:16: error: base class 'rocksdb::LDBCommand' is uninitialized when used here to access 'rocksdb::LDBCommand::BuildCmdLineOptions' [-Werror,-Wuninitialized]
               BuildCmdLineOptions({ARG_FROM, ARG_TO, ARG_HEX, ARG_KEY_HEX,
               ^
...
```

```lang=c++
CompactorCommand::CompactorCommand(const vector<string>& params,
      const map<string, string>& options, const vector<string>& flags) :
    LDBCommand(options, flags, false,
               BuildCmdLineOptions({ARG_FROM, ARG_TO, ARG_HEX, ARG_KEY_HEX,
                                    ARG_VALUE_HEX, ARG_TTL})),
    null_from_(true), null_to_(true) {
. . .
}
```
For the fourth parameter of the base constructor (`LDBCommand`) we call `BuildCmdLineOptions`, which is a private non-static method of `LDBCommand` base class.

This diff adds missing `static` keyword for `LDBCommand::BuildCmdLineOptions` method.

Test Plan:
Build with trunk clang and make sure all tests are passing.
```lang=bash
% # Have trunk clang present in path.
% ROCKSDB_NO_FBCODE=1 CC=clang CXX=clang++ make check
``

Reviewers: meyering, sdong, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D34083

util/ldb_cmd.h

@@ -290,7 +290,7 @@ protected:
    * used by this command.  It includes the common options and the ones
    * passed in.
    */
-  vector<string> BuildCmdLineOptions(vector<string> options) {
+  static vector<string> BuildCmdLineOptions(vector<string> options) {
     vector<string> ret = {ARG_DB,               ARG_BLOOM_BITS,
                           ARG_BLOCK_SIZE,       ARG_AUTO_COMPACTION,
                           ARG_COMPRESSION_TYPE, ARG_WRITE_BUFFER_SIZE,