Fix implicit conversion changes signess warning in perf_counters.cc

Newly added -Wconversion command line option make this warning to stop
compilation with error, because warnings are treated as errors.

src/perf_counters.cc

@@ -39,7 +39,8 @@ size_t PerfCounterValues::Read(const std::vector<int>& leaders) {
     auto read_bytes = ::read(lead, ptr, size);
     if (read_bytes >= ssize_t(sizeof(uint64_t))) {
       // Actual data bytes are all bytes minus initial padding
-      std::size_t data_bytes = read_bytes - sizeof(uint64_t);
+      std::size_t data_bytes =
+          static_cast<std::size_t>(read_bytes) - sizeof(uint64_t);
       // This should be very cheap since it's in hot cache
       std::memmove(ptr, ptr + sizeof(uint64_t), data_bytes);
       // Increment our counters