tint: Fix include layering violation

Headers exposing functionality from the core Tint library should be
included via include/tint/tint.h.

This fixes the google3 build.

Change-Id: I82a9d0105b3b44fb4c4c89b59e9282290dd49c61
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/93120
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: James Price <jrprice@google.com>

include/tint/tint.h

@@ -33,6 +33,7 @@
 #include "src/tint/transform/robustness.h"
 #include "src/tint/transform/single_entry_point.h"
 #include "src/tint/transform/vertex_pulling.h"
+#include "src/tint/writer/flatten_bindings.h"
 #include "src/tint/writer/writer.h"
 
 #if TINT_BUILD_SPV_READER

src/tint/cmd/main.cc

@@ -34,7 +34,6 @@
 #include "src/tint/utils/io/command.h"
 #include "src/tint/utils/string.h"
 #include "src/tint/val/val.h"
-#include "src/tint/writer/flatten_bindings.h"
 #include "tint/tint.h"
 
 namespace {