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/timing.h b/include/mbedtls/timing.h
index bbcb906..a965fe0 100644
--- a/include/mbedtls/timing.h
+++ b/include/mbedtls/timing.h
@@ -51,7 +51,7 @@
 /**
  * \brief          Context for mbedtls_timing_set/get_delay()
  */
-typedef struct
+typedef struct mbedtls_timing_delay_context
 {
     struct mbedtls_timing_hr_time   timer;
     uint32_t                        int_ms;