chore(clippy): deny clippy::dbg_macro

I noticed that I had let `dbg!(…)` sneak into commits in gfx-rs#2294 by
accident. Clippy can help us with this! Set `deny(clippy::dbg_macro)`,
so CI catches this for future contributions.

src/lib.rs

@@ -215,7 +215,7 @@ tree.
     clippy::rest_pat_in_fully_bound_structs,
     clippy::match_wildcard_for_single_variants
 )]
-#![cfg_attr(not(test), deny(clippy::panic))]
+#![cfg_attr(not(test), deny(clippy::dbg_macro, clippy::panic))]
 
 mod arena;
 pub mod back;