Add CI to check that Ruby headers are up-to-date

.github/workflows/header_check.yml

@@ -0,0 +1,15 @@
+name: Header check
+on: [push, pull_request]
+
+jobs:
+  header_check:
+    runs-on: ubuntu-latest
+    name: Check vendored Ruby headers
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: "3.3"
+          bundler-cache: true
+      - run: bundle exec rake vendor_ruby_headers
+      - run: git diff --no-ext-diff --ignore-submodules --exit-code

gc/gc_impl.h

@@ -106,6 +106,7 @@ GC_IMPL_FN size_t rb_gc_impl_gc_count(void *objspace_ptr);
 GC_IMPL_FN VALUE rb_gc_impl_latest_gc_info(void *objspace_ptr, VALUE key);
 GC_IMPL_FN VALUE rb_gc_impl_stat(void *objspace_ptr, VALUE hash_or_sym);
 GC_IMPL_FN VALUE rb_gc_impl_stat_heap(void *objspace_ptr, VALUE heap_name, VALUE hash_or_sym);
+GC_IMPL_FN const char *rb_gc_impl_active_gc_name(void);
 // Miscellaneous
 GC_IMPL_FN size_t rb_gc_impl_obj_flags(void *objspace_ptr, VALUE obj, ID* flags, size_t max);
 GC_IMPL_FN bool rb_gc_impl_pointer_to_heap_p(void *objspace_ptr, const void *ptr);