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/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index f404f89..9237165 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -2,6 +2,7 @@
#include "mbedtls/bignum.h"
#include "mbedtls/x509_crt.h"
#include "mbedtls/x509_csr.h"
+#include "mbedtls/x509_internal.h"
#include "mbedtls/pem.h"
#include "mbedtls/oid.h"
#include "mbedtls/rsa.h"