auth-api: cleanup a warning

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

proxmox-auth-api/src/auth_key.rs

@@ -208,6 +208,7 @@ impl HMACKey {
     // This is needed for legacy CSRF token verifyication.
     //
     // TODO: remove once all dependent products had a major version release (PBS)
+    #[cfg(feature = "api")]
     pub(crate) fn as_bytes(&self) -> Result<Vec<u8>, Error> {
         // workaround to get access to the the bytes behind the key.
         self.key