Files that are #included in a non-standard header way usually have a .inc or .inl extension.
Reactor already has Reactor/Traits.inl, so let's be consistent and stick with this extension.

Bug: b/148778720
Change-Id: Ie69951c254921b9cf86c3ab201533a3b21489b97
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40977
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Ben Clayton <bclayton@google.com>