[gen.rs] Improve mk_ptrty.
[gen.rs] Improve mk_ptrty.

Twinklebear/rust-bindgen,Twinklebear/rust-bindgen,Twinklebear/rust-bindgen,Twinklebear/rust-bindgen,bsteinb/rust-bindgen,alexander255/rust-bindgen,alexander255/rust-bindgen,bsteinb/rust-bindgen,bsteinb/rust-bindgen,alexander255/rust-bindgen