x509: move internal functions declarations to a private header

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index 503d976..765866b 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 "x509_internal.h"
 #include "mbedtls/pem.h"
 #include "mbedtls/oid.h"
 #include "mbedtls/rsa.h"