include private pk.h internally

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/programs/pkey/pk_verify.c b/programs/pkey/pk_verify.c
index d9e3bf1..507812e 100644
--- a/programs/pkey/pk_verify.c
+++ b/programs/pkey/pk_verify.c
@@ -26,6 +26,9 @@
 #else
 
 #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>