Add an HKDF (RFC 5869) implementation
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 6177ca2..b730d08 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -29,6 +29,7 @@
     error.c
     gcm.c
     havege.c
+    hkdf.c
     hmac_drbg.c
     md.c
     md2.c