[wasm][cleanup] Remove include of src/signature.h in wasm-opcodes.h

Bug: v8:7570

Change-Id: Ib3ff7e64390b1f159905a054e7f9e37bd3f8a19c
Reviewed-on: https://chromium-review.googlesource.com/1021530
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52706}

src/wasm/wasm-opcodes.h

@@ -8,11 +8,14 @@
 #include "src/globals.h"
 #include "src/machine-type.h"
 #include "src/runtime/runtime.h"
-#include "src/signature.h"
 #include "src/wasm/wasm-constants.h"
 
 namespace v8 {
 namespace internal {
+
+template <typename T>
+class Signature;
+
 namespace wasm {
 
 // We reuse the internal machine type to represent WebAssembly types.