Ignore buggy clippy::incompatible_msrv lint

```
error: current MSRV (Minimum Supported Rust Version) is `1.31.0` but this item is stable since `1.37.0`
   --> src/lib.rs:885:32
    |
885 |         let ptr = self.entries.as_mut_ptr();
    |                                ^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
    = note: `-D clippy::incompatible-msrv` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::incompatible_msrv)]`

error: current MSRV (Minimum Supported Rust Version) is `1.31.0` but this item is stable since `1.37.0`
   --> src/lib.rs:934:37
    |
934 |         let base_ptr = self.entries.as_ptr() as usize;
    |                                     ^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
```

src/lib.rs

@@ -9,6 +9,7 @@
     no_crate_inject,
     attr(deny(warnings, rust_2018_idioms), allow(dead_code, unused_variables))
 ))]
+#![allow(clippy::incompatible_msrv)] // false positive: https://github.com/rust-lang/rust-clippy/issues/12280
 
 //! Pre-allocated storage for a uniform data type.
 //!