Add #include for hmac.h

This fix has already been applied upstream in
66d14e4d7f764bb07cb68ef274e5599d8671cb4d.
diff --git a/c/ssl_private.h b/c/ssl_private.h
index e101f08..9cfe24a 100644
--- a/c/ssl_private.h
+++ b/c/ssl_private.h
@@ -56,6 +56,7 @@
 #include <openssl/opensslv.h>
 #include <openssl/ssl.h>
 #include <openssl/err.h>
+#include <openssl/hmac.h>
 #include <openssl/x509.h>
 #include <openssl/pem.h>
 #include <openssl/pkcs12.h>