[build] Enable external flag header by default

Turns on v8_generate_external_defines_header.

Bug: v8:11292
Change-Id: I4b1d9b47390b560b7cbf677948310694d8b03367
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2610966Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72201}

BUILD.gn

@@ -288,7 +288,7 @@ declare_args() {
 
   # If enabled then macro definitions that are used in externally visible
   # header files are placed in a separate header file v8-gn.h.
-  v8_generate_external_defines_header = false
+  v8_generate_external_defines_header = true
 }
 
 # Derived defaults.