MSVC: Fix narrowing conversion error

MSVC15 compiler requires an explicit
coversion from int32_t to uint32_t

TraceTimestamp {
  uint32_t clock_id,
  uint64_t_value
}
while GetClockId() is enum BuiltinClock : int32_t

Since all values of enum BuiltinClock are >=0 explicit cast should
not bring any additional problems and is done on most of compilers
automatically anyway

Change-Id: Iad034fbe389beb8739ff03aa1751e03d351a8105
diff --git a/include/perfetto/tracing/internal/track_event_data_source.h b/include/perfetto/tracing/internal/track_event_data_source.h
index 4f083a8..9b64708 100644
--- a/include/perfetto/tracing/internal/track_event_data_source.h
+++ b/include/perfetto/tracing/internal/track_event_data_source.h
@@ -53,7 +53,7 @@
 struct TraceTimestampTraits<uint64_t> {
   static inline TraceTimestamp ConvertTimestampToTraceTimeNs(
       const uint64_t& timestamp) {
-    return {internal::TrackEventInternal::GetClockId(), timestamp};
+    return {static_cast<uint32_t>(internal::TrackEventInternal::GetClockId()), timestamp};
   }
 };