Add wrappers for XPLMGetSystemPath

[JDeeth]

Currently there's no wrapper for XPLMGetSystemPath

https://developer.x-plane.com/sdk/XPLMGetSystemPath/ https://docs.rs/xplm-sys/latest/xplm_sys/fn.XPLMGetSystemPath.html

I'm a bit unsure how to use this function safely in Rust given it's a string pointer outparam!