Allow to forward declare of public structs #1215

Thanks to forward declare we can declare `struct` in our header file instead making #include
diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index ac23cff..d41ec93 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -105,7 +105,7 @@
  *
  * All lists are bitfields, built by ORing flags from MBEDTLS_X509_ID_FLAG().
  */
-typedef struct
+typedef struct mbedtls_x509_crt_profile
 {
     uint32_t allowed_mds;       /**< MDs for signatures         */
     uint32_t allowed_pks;       /**< PK algs for signatures     */