x509: move internal functions declarations to a private header

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/x509.c b/library/x509.c
index b7b71f3..f97fb44 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -19,7 +19,7 @@
 
 #if defined(MBEDTLS_X509_USE_C)
 
-#include "mbedtls/x509.h"
+#include "x509_internal.h"
 #include "mbedtls/asn1.h"
 #include "mbedtls/error.h"
 #include "mbedtls/oid.h"