Make have_basepri an expected cfg

Since Rust 1.80 there is automatic compile-time checks for unexpected
cfgs.

Ensure that `have_basepri` cfg is known by the compiler emitting it in
the build.rs.

CHANGELOG.md

@@ -11,6 +11,8 @@ For each category, *Added*, *Changed*, *Fixed* add new entries at the top!
 
 ### Fixed
 
+- CFG: Make `have_basepri` an expected cfg
+
 ### Changed
 
 ## [v1.1.4] - 2023-02-26

build.rs

@@ -7,6 +7,9 @@ fn main() {
         println!("cargo:rustc-cfg=rustc_is_nightly");
     }
 
+    // Make `have_basepri` an expected cfg.
+    println!("cargo::rustc-check-cfg=cfg(have_basepri)");
+
     // These targets all have know support for the BASEPRI register.
     if target.starts_with("thumbv7m")
         | target.starts_with("thumbv7em")