Build db_bench_log executable

[hanlins]

This change added the db_bench_log executable to the build.

[hanlins]

Seems db_bench_log was missing:

➜  leveldb git:(main) find . -name "db_bench*.cc" 
./benchmarks/db_bench_sqlite3.cc
./benchmarks/db_bench.cc
./benchmarks/db_bench_tree_db.cc
./benchmarks/db_bench_log.cc
➜  leveldb git:(main) grep "db_bench.*.cc" CMakeLists.txt
    leveldb_benchmark("benchmarks/db_bench.cc")
    leveldb_benchmark("benchmarks/db_bench_sqlite3.cc")
    leveldb_benchmark("benchmarks/db_bench_tree_db.cc")

Tested manually and build works:

➜  leveldb git:(topic/build-db_bench_log) cmake --build build --config Debug --target db_bench_log 
[1/1] Linking CXX executable db_bench_log
➜  leveldb git:(topic/build-db_bench_log) echo $?
0