Add hkdf.h to doc.config.

It has all of one function in there.

Change-Id: I86f0fbb76d267389c62b63ac01df685acb70535e
Reviewed-on: https://boringssl-review.googlesource.com/7723
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/hkdf.h

diff --git a/include/openssl/hkdf.h b/include/openssl/hkdf.h
index b7a0dc2..8c96c4c 100644
--- a/include/openssl/hkdf.h
+++ b/include/openssl/hkdf.h
@@ -17,14 +17,17 @@
 
 #include <openssl/base.h>
 
-#ifdef  __cplusplus
+#if defined(__cplusplus)
 extern "C" {
 #endif
 
 
-/* Computes HKDF (as specified by RFC 5869) of initial keying material |secret|
- * with |salt| and |info| using |digest|, and outputs |out_len| bytes to
- * |out_key|. It returns one on success and zero on error.
+/* HKDF. */
+
+
+/* HKDF computes HKDF (as specified by RFC 5869) of initial keying material
+ * |secret| with |salt| and |info| using |digest|, and outputs |out_len| bytes
+ * to |out_key|. It returns one on success and zero on error.
  *
  * HKDF is an Extract-and-Expand algorithm. It does not do any key stretching,
  * and as such, is not suited to be used alone to generate a key from a

util/doc.config

diff --git a/util/doc.config b/util/doc.config
index cbee543..af02029 100644
--- a/util/doc.config
+++ b/util/doc.config
@@ -33,6 +33,7 @@
       "include/openssl/ecdh.h",
       "include/openssl/ecdsa.h",
       "include/openssl/engine.h",
+      "include/openssl/hkdf.h",
       "include/openssl/hmac.h",
       "include/openssl/md5.h",
       "include/openssl/rc4.h",