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/oid.h b/include/mbedtls/oid.h
index 408645e..bd4f179 100644
--- a/include/mbedtls/oid.h
+++ b/include/mbedtls/oid.h
@@ -388,7 +388,8 @@
 /**
  * \brief Base OID descriptor structure
  */
-typedef struct {
+typedef struct mbedtls_oid_descriptor_t
+{
     const char *asn1;               /*!< OID ASN.1 representation       */
     size_t asn1_len;                /*!< length of asn1                 */
     const char *name;               /*!< official name (e.g. from RFC)  */