Include hmac.h in crypto_util.h.

`crypto_util.h` references `HMAC_CTX_free` but doesn't include the
header file that contains it.

Author: agl

src/crypto/crypto_util.h

@@ -15,6 +15,7 @@
 #include <openssl/err.h>
 #include <openssl/evp.h>
 #include <openssl/ec.h>
+#include <openssl/hmac.h>
 #include <openssl/kdf.h>
 #include <openssl/rsa.h>
 #include <openssl/dsa.h>