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/platform.h b/include/mbedtls/platform.h
index 624cc64..a40a64f 100644
--- a/include/mbedtls/platform.h
+++ b/include/mbedtls/platform.h
@@ -315,7 +315,8 @@
  * \note    This structure may be used to assist platform-specific
  *          setup or teardown operations.
  */
-typedef struct {
+typedef struct mbedtls_platform_context
+{
     char dummy; /**< A placeholder member, as empty structs are not portable. */
 }
 mbedtls_platform_context;