Add missing include of type_traits

Recent libc++ versions no longer implicitly include <type_traits>
as part of <utility>, see https://reviews.llvm.org/D140426

Bug: chromium:1404923
Change-Id: Ic688b0ca05167ccae1db916ed964b77f53ff999f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/116283
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

src/tint/traits.h

@@ -16,6 +16,7 @@
 #define SRC_TINT_TRAITS_H_
 
 #include <tuple>
+#include <type_traits>
 #include <utility>
 
 namespace tint::traits {