Add cstddef for definition of size_t

Change-Id: Ic92072a680951145f4925a2ac640f74a8f8b4335
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/72640
Auto-Submit: David Neto <dneto@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
diff --git a/src/utils/unique_vector.h b/src/utils/unique_vector.h
index f64ed07..1c3fc4d 100644
--- a/src/utils/unique_vector.h
+++ b/src/utils/unique_vector.h
@@ -15,6 +15,7 @@
 #ifndef SRC_UTILS_UNIQUE_VECTOR_H_
 #define SRC_UTILS_UNIQUE_VECTOR_H_
 
+#include <cstddef>
 #include <functional>
 #include <unordered_set>
 #include <utility>