Change v8_wrappers to v8_header_set

The wrappers target currently only has one header. Marking it as source
set created trouble on Windows and Mac builders.

Bug: v8:11303
Change-Id: I228d49c4150eb089e8494555886072b237b95940
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2626855Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: John Xu <johnx@google.com>
Cr-Commit-Position: refs/heads/master@{#72089}

BUILD.gn

@@ -2021,7 +2021,7 @@ v8_header_set("v8_headers") {
   deps = [ ":v8_version" ]
 }
 
-v8_source_set("v8_wrappers") {
+v8_header_set("v8_wrappers") {
   configs = [ ":internal_config" ]
 
   sources = [ "src/base/platform/wrappers.h" ]