Add minimalistic HMAC_DRBG implementation

(copied from ECDSA)
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index dd6bbd3..77aac20 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -26,6 +26,7 @@
      error.c
      gcm.c
      havege.c
+     hmac_drbg.c
      md.c
      md_wrap.c
      md2.c