Remove OPENSSL_EXPORT from enum in fillins/net_errors.h

Windows in chrome also does not like this

bug: chromium:1322914
Change-Id: I79c788e0b521964fdc07b530ec47d7fc3635e5a4
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/62765
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Auto-Submit: Bob Beck <bbe@google.com>

pki/fillins/net_errors.h

@@ -9,7 +9,7 @@
 
 namespace bssl {
 
-enum OPENSSL_EXPORT Error {
+enum Error {
   OK = 0,
 };