Add header for std::forward

Previously this was being indirectly supplied by <memory>, which has
been removed. Clang happily still finds std::forward, but GCC does
not.

BUG=dawn:768

Change-Id: I87e6bb6aa83bc7f3253c58e32fa0ba977752858c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/48820
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/common/SlabAllocator.h

diff --git a/src/common/SlabAllocator.h b/src/common/SlabAllocator.h
index 7d7a690..8a78dde 100644
--- a/src/common/SlabAllocator.h
+++ b/src/common/SlabAllocator.h
@@ -19,6 +19,7 @@
 
 #include <cstdint>
 #include <type_traits>
+#include <utility>
 
 // The SlabAllocator allocates objects out of one or more fixed-size contiguous "slabs" of memory.
 // This makes it very quick to allocate and deallocate fixed-size objects because the allocator only