x509-cert: builder: implement std::error::Error for Error

Without this, x509-cert::builder::Error can not be used in the pattern
of Box<dyn Error>, which is pretty common.

Signed-off-by: Dmitry Baryshkov <[email protected]>

Author: lumag

x509-cert/src/builder.rs

@@ -36,6 +36,9 @@ pub enum Error {
     Signature(signature::Error),
 }
 
+#[cfg(feature = "std")]
+impl std::error::Error for Error {}
+
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {