[build] fix static library build

Bug: v8:7784
Change-Id: Ie4bbecacc9bced842269871dd5efa5113e09059d
Reviewed-on: https://chromium-review.googlesource.com/1070200Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53307}

BUILD.gn

@@ -905,7 +905,7 @@ action("run_torque") {
   }
 }
 
-v8_source_set("torque_generated_core") {
+v8_header_set("torque_generated_core") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
   deps = [