Use partial path to benchmark/benchmark.h.

Using the partial path offers more flexibility to projects which
may checkout google/benchmark to a different location.

PiperOrigin-RevId: 357819911

db/db_test.cc

@@ -9,7 +9,7 @@
 #include <string>
 
 #include "gtest/gtest.h"
-#include "third_party/benchmark/include/benchmark/benchmark.h"
+#include "benchmark/benchmark.h"
 #include "db/db_impl.h"
 #include "db/filename.h"
 #include "db/version_set.h"