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_pkcs7.function b/tests/suites/test_suite_pkcs7.function
index 65384a8..4c8bf23 100644
--- a/tests/suites/test_suite_pkcs7.function
+++ b/tests/suites/test_suite_pkcs7.function
@@ -4,6 +4,7 @@
 #include "mbedtls/x509.h"
 #include "mbedtls/x509_crt.h"
 #include "mbedtls/x509_crl.h"
+#include "x509_internal.h"
 #include "mbedtls/oid.h"
 #include "sys/types.h"
 #include "sys/stat.h"