Add some IWYU export pragmas

We split nid.h out of obj.h and evp_errors.h out of evp.h, but folks who
include the original headers can reasonably assume that the child header
is included.

Change-Id: I81c40fd88df58500a0f10bfa864b8bc98451dbc0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/63485
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

include/openssl/evp.h

diff --git a/include/openssl/evp.h b/include/openssl/evp.h
index 6f6eaa9..8a9d620 100644
--- a/include/openssl/evp.h
+++ b/include/openssl/evp.h
@@ -59,7 +59,7 @@
 
 #include <openssl/base.h>
 
-#include <openssl/evp_errors.h>
+#include <openssl/evp_errors.h>  // IWYU pragma: export
 #include <openssl/thread.h>
 
 // OpenSSL included digest and cipher functions in this header so we include

include/openssl/obj.h

diff --git a/include/openssl/obj.h b/include/openssl/obj.h
index 51c5b30..57ea397 100644
--- a/include/openssl/obj.h
+++ b/include/openssl/obj.h
@@ -60,7 +60,7 @@
 #include <openssl/base.h>
 
 #include <openssl/bytestring.h>
-#include <openssl/nid.h>
+#include <openssl/nid.h>  // IWYU pragma: export
 
 #if defined(__cplusplus)
 extern "C" {