Move declarations of internal X.509 functions to separate header
This makes it easier to distinguish public from internal functions;
for us, for users, and for automated API compatibility checkers.
diff --git a/library/x509_create.c b/library/x509_create.c
index 546e8fa..1639630 100644
--- a/library/x509_create.c
+++ b/library/x509_create.c
@@ -28,6 +28,7 @@
#if defined(MBEDTLS_X509_CREATE_C)
#include "mbedtls/x509.h"
+#include "mbedtls/x509_internal.h"
#include "mbedtls/asn1write.h"
#include "mbedtls/oid.h"