Make the pki private headers available to gn

So the chrome build can be slightly less oopy

Bug: chromium:1322914

Change-Id: I0967ba67f8e43e8b996bc246c516b9ffe50bc708
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/63665
Auto-Submit: Bob Beck <bbe@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

util/generate_build_files.py

diff --git a/util/generate_build_files.py b/util/generate_build_files.py
index 9f24a2b..e4e95ca 100644
--- a/util/generate_build_files.py
+++ b/util/generate_build_files.py
@@ -359,7 +359,9 @@
                                 files['ssl'] + files['ssl_internal_headers'])
       self.PrintVariableSection(out, 'ssl_headers', files['ssl_headers'])
       self.PrintVariableSection(out, 'pki_sources',
-                                files['pki'] + files['pki_internal_headers'])
+                                files['pki'])
+      self.PrintVariableSection(out, 'pki_internal_headers',
+                                files['pki_internal_headers'])
       self.PrintVariableSection(out, 'tool_sources',
                                 files['tool'] + files['tool_headers'])