Fix RocksDB lite build

Summary:
NewMemEnv() is defined in rocksdb lite but just returns nullptr --
would it be better to just not define it so we can catch issues like this at
compile-time?

Test Plan:
  $ make clean && OPT="-DTRAVIS -DROCKSDB_LITE" V=1 make -j32 db_test
  $ ./db_test --gtest_filter='DBTest.MemEnvTest'
  ...
  [  PASSED  ] 0 tests.

Reviewers: yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D53427

db/db_test.cc

@@ -169,6 +169,9 @@ TEST_F(DBTest, MockEnvTest) {
   delete db;
 }
 
+// NewMemEnv returns nullptr in ROCKSDB_LITE since class InMemoryEnv isn't
+// defined.
+#ifndef ROCKSDB_LITE
 TEST_F(DBTest, MemEnvTest) {
   unique_ptr<Env> env{NewMemEnv(Env::Default())};
   Options options;
@@ -221,6 +224,7 @@ TEST_F(DBTest, MemEnvTest) {
   }
   delete db;
 }
+#endif  // ROCKSDB_LITE
 
 TEST_F(DBTest, WriteEmptyBatch) {
   Options options;