Add missing Constants.h in src/common/BUILD.gn

This patch adds the missing Constants.h in src/common/BUILD.gn so that
we can see this file under the folder "src/common" of the Visual Studio
project generated by GN.

BUG=dawn:134

Change-Id: I513315ee2adbd93cf1ff61d112e8a4f65f660bce
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/6520
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/common/BUILD.gn

@@ -79,6 +79,7 @@ if (is_win || is_linux || is_mac) {
       "Assert.h",
       "BitSetIterator.h",
       "Compiler.h",
+      "Constants.h",
       "DynamicLib.cpp",
       "DynamicLib.h",
       "HashUtils.h",