x509: move internal functions declarations to a private header

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/x509write_csr.c b/library/x509write_csr.c
index 254da69..0a36202 100644
--- a/library/x509write_csr.c
+++ b/library/x509write_csr.c
@@ -14,7 +14,7 @@
 
 #if defined(MBEDTLS_X509_CSR_WRITE_C)
 
-#include "mbedtls/x509.h"
+#include "x509_internal.h"
 #include "mbedtls/x509_csr.h"
 #include "mbedtls/asn1write.h"
 #include "mbedtls/error.h"