Bug 1656945 - use <type_traits> in TelemetryHistogramEnums.h; r=Dexter

This header may have used `mozilla/TemplateLib.h` at sometime in the distant
past, but it doesn't anymore; let's use the correct header instead.

Differential Revision: https://phabricator.services.mozilla.com/D85775

toolkit/components/telemetry/build_scripts/gen_histogram_enum.py

@@ -28,7 +28,7 @@
 #ifndef mozilla_TelemetryHistogramEnums_h
 #define mozilla_TelemetryHistogramEnums_h
 
-#include "mozilla/TemplateLib.h"
+#include <type_traits>
 
 namespace mozilla {
 namespace Telemetry {