[heap] Fix compilation error on gcc

After https://crrev.com/c/3211894 the following error
gets thrown on gcc:
```
 error: call to non-'constexpr' function 'uint8_t
v8::internal::LocalHeap::ThreadState::raw() const'
         : raw_state_(state.raw()) {}
```

Bug: v8:11708
Change-Id: I6377c95fa38d4b4670f6a513e061f13e349a3212
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3216043Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Milad Fa <mfarazma@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#77369}

src/heap/local-heap.h

@@ -189,7 +189,7 @@ class V8_EXPORT_PRIVATE LocalHeap {
    private:
     constexpr explicit ThreadState(uint8_t value) : raw_state_(value) {}
 
-    uint8_t raw() const { return raw_state_; }
+    constexpr uint8_t raw() const { return raw_state_; }
 
     uint8_t raw_state_;