Add private include to additional files

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/library/x509_oid.h b/library/x509_oid.h
index c2fe8dc..8d5e1bb 100644
--- a/library/x509_oid.h
+++ b/library/x509_oid.h
@@ -13,6 +13,9 @@
 
 #include "mbedtls/asn1.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 "mbedtls/x509.h"
 
 #include <stddef.h>