Add private include to additional files

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 9228a3b..a462a07 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -42,6 +42,9 @@
 #endif
 
 #include "mbedtls/pk.h"
+#if defined(MBEDTLS_PK_HAVE_PRIVATE_HEADER)
+#include <mbedtls/private/pk_private.h>
+#endif /* MBEDTLS_PK_HAVE_PRIVATE_HEADER */
 #include "ssl_ciphersuites_internal.h"
 #include "x509_internal.h"
 #include "pk_internal.h"