`reaper_plugin_functions.h` is not kept up-to-date

[cfillion]

The copy on this repository hasn't been updated in 3 years and is missing newer functions such as realloc_cmd_register_buf.

It would be useful if the SDK was always kept in sync (eg. automatically) with new REAPER releases, so that it's easier to pull fresh headers with git (versus having to manually generate it from the GUI REAPER action and maintain a local copy of that file).