Fix initializer syntax for old Xcode compiler (#6662)

Summary:
Example compiler output, from OSX TEST_GROUP=3:

db/flush_job_test.cc:185:7: error: suggest braces around initialization
of subobject [-Werror,-Wmissing-braces]
      kInvalidBlobFileNumber, 5, 103, 17, 102, 101};

Apparently permitted in newer version, but worth working around.
https://stackoverflow.com/questions/31555584/why-is-clang-warning-suggest-braces-around-initialization-of-subobject-wmis
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6662

Test Plan: CI (temporarily including OSX TEST_GROUP=3 in Travis)

Reviewed By: ltamasi

Differential Revision: D20901009

Pulled By: pdillinger

fbshipit-source-id: 5338878613b5725e5d632c8858904de467dc4692

db/flush_job_test.cc

@@ -181,8 +181,8 @@ TEST_F(FlushJobTest, NonEmpty) {
   // Note: the first two blob references will not be considered when resolving
   // the oldest blob file referenced (the first one is inlined TTL, while the
   // second one is TTL and thus points to a TTL blob file).
-  constexpr std::array<uint64_t, 6> blob_file_numbers{
+  constexpr std::array<uint64_t, 6> blob_file_numbers{{
-      kInvalidBlobFileNumber, 5, 103, 17, 102, 101};
+      kInvalidBlobFileNumber, 5, 103, 17, 102, 101}};
   for (size_t i = 0; i < blob_file_numbers.size(); ++i) {
     std::string key(ToString(i + 10001));
     std::string blob_index;