Fix doc build

The addition of the KRB5_PADATA_AS_CHECKSUM macro in d7d7486 caused
the doxygen bridge to emit a new RST file.  This file was not included in
the API reference toctree, causing a build failure in maintainer-mode.

doc/appdev/refs/macros/index.rst

@@ -232,6 +232,7 @@ Public
    KRB5_PAC_UPN_DNS_INFO.rst
    KRB5_PADATA_AFS3_SALT.rst
    KRB5_PADATA_AP_REQ.rst
+   KRB5_PADATA_AS_CHECKSUM.rst
    KRB5_PADATA_ENCRYPTED_CHALLENGE.rst
    KRB5_PADATA_ENC_SANDIA_SECURID.rst
    KRB5_PADATA_ENC_TIMESTAMP.rst