Fix compiler warnings (#3784)

The definition of os_get_invalid_handle in platform_internal.h did not
match the declaration in platform_api_extension.h.

core/shared/platform/riot/platform_internal.h

@@ -89,7 +89,7 @@ int isnan(double x);
 #endif
 
 static inline os_file_handle
-os_get_invalid_handle()
+os_get_invalid_handle(void)
 {
     return -1;
 }