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/library/x509_crt.c b/library/x509_crt.c
index 9004be4..0f72e2f 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -40,6 +40,7 @@
#if defined(MBEDTLS_X509_CRT_PARSE_C)
#include "mbedtls/x509_crt.h"
+#include "mbedtls/x509_internal.h"
#include "mbedtls/oid.h"
#include "mbedtls/platform_util.h"