Conditionally include ALIGN_TO

This const is only used under specific features, use `cfg` to
conditionally build it in.

Removes clippy warning.

secp256k1-sys/src/types.rs

@@ -40,6 +40,7 @@ impl AlignedType {
     }
 }
 
+#[cfg(all(feature = "std", not(feature = "external-symbols")))]
 pub(crate) const ALIGN_TO: usize = mem::align_of::<AlignedType>();