Uncomment one println in build.rs

Even though this cfg isn't currently used, printing it doesn't hurt, and
it avoids a Clippy lint

Author: jonas-schievink

build.rs

@@ -25,7 +25,7 @@ fn main() {
     } else if target.starts_with("thumbv7em-") {
         println!("cargo:rustc-cfg=cortex_m");
         println!("cargo:rustc-cfg=armv7m");
-        //println!("cargo:rustc-cfg=armv7em");
+        println!("cargo:rustc-cfg=armv7em");  // (not currently used)
     } else if target.starts_with("thumbv8m.base") {
         println!("cargo:rustc-cfg=cortex_m");
         println!("cargo:rustc-cfg=armv8m");