Add Multiplanar External Texture Transform To Tint Header

Adds src/transform/multiplanar_external_texture.h to the Tint header.
This is needed to access the MultiplanarExternalTexture::BindingsMap
type from Dawn.

Bug: dawn:1082
Change-Id: Ie6e14e3553af96dc0110bf95bc6a4fc76dcea26d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/76120
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/include/tint/tint.h b/include/tint/tint.h
index a28d85e..f959cbf 100644
--- a/include/tint/tint.h
+++ b/include/tint/tint.h
@@ -28,6 +28,7 @@
 #include "src/transform/first_index_offset.h"
 #include "src/transform/fold_trivial_single_use_lets.h"
 #include "src/transform/manager.h"
+#include "src/transform/multiplanar_external_texture.h"
 #include "src/transform/renamer.h"
 #include "src/transform/robustness.h"
 #include "src/transform/single_entry_point.h"