bpf_benchmark.cpp - fix a clang warning: clang-analyzer-deadcode.DeadStores

system/netd/tests/benchmarks/bpf_benchmark.cpp:37:15: warning: Value stored to '_' during its initialization is never read [clang-analyzer-deadcode.DeadStores]
    for (auto _ : state) {
              ^
system/netd/tests/benchmarks/bpf_benchmark.cpp:37:15: note: Value stored to '_' during its initialization is never read

system/netd/tests/benchmarks/bpf_benchmark.cpp:48:15: warning: Value stored to '_' during its initialization is never read [clang-analyzer-deadcode.DeadStores]
    for (auto _ : state) {
              ^
system/netd/tests/benchmarks/bpf_benchmark.cpp:48:15: note: Value stored to '_' during its initialization is never read

system/netd/tests/benchmarks/bpf_benchmark.cpp:59:15: warning: Value stored to '_' during its initialization is never read [clang-analyzer-deadcode.DeadStores]
    for (auto _ : state) {
              ^
system/netd/tests/benchmarks/bpf_benchmark.cpp:59:15: note: Value stored to '_' during its initialization is never read

system/netd/tests/benchmarks/bpf_benchmark.cpp:68:15: warning: Value stored to '_' during its initialization is never read [clang-analyzer-deadcode.DeadStores]
    for (auto _ : state) {
              ^
system/netd/tests/benchmarks/bpf_benchmark.cpp:68:15: note: Value stored to '_' during its initialization is never read

Test: builds
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I53ba19ebd8f87d4a3f4649b0d3e0a22f0d204b42