build: Make cppgc_base a public dependency

Similar to V8, we provide the include/ headers through a "base" target.

Without this change, `gn check` complains about including cppgc headers
in Blink.

Bug: chromium:1056170
Change-Id: I09ad943cdfedae8c83ab7f957efba796637b8d48
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2617087Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71979}

BUILD.gn

@@ -3858,7 +3858,6 @@ v8_source_set("v8_base_without_compiler") {
   ]
 
   deps = [
-    ":cppgc_base",
     ":torque_generated_definitions",
     ":v8_cppgc_shared",
     ":v8_headers",
@@ -3872,6 +3871,7 @@ v8_source_set("v8_base_without_compiler") {
   ]
 
   public_deps = [
+    ":cppgc_base",
     ":generate_bytecode_builtins_list",
     ":run_torque",
     ":v8_maybe_icu",