x509: move internal functions declarations to a private header

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 7cbc6af..101b204 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -45,6 +45,7 @@
 
 #include "mbedtls/pk.h"
 #include "ssl_ciphersuites_internal.h"
+#include "x509_internal.h"
 #include "pk_internal.h"
 #include "common.h"