include private pk.h internally

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/programs/pkey/rsa_sign_pss.c b/programs/pkey/rsa_sign_pss.c
index 94333ae..8f605b5 100644
--- a/programs/pkey/rsa_sign_pss.c
+++ b/programs/pkey/rsa_sign_pss.c
@@ -31,6 +31,9 @@
 #include "mbedtls/ctr_drbg.h"
 #include "mbedtls/rsa.h"
 #include "mbedtls/pk.h"
+#if defined(MBEDTLS_PK_HAVE_PRIVATE_HEADER)
+#include <mbedtls/private/pk_private.h>
+#endif /* MBEDTLS_PK_HAVE_PRIVATE_HEADER */
 
 #include <stdio.h>
 #include <string.h>