Fix build with poppler <0.58

Summary:
- Poppler::FormFieldSignature::signatureType
- Poppler::SignatureValidationInfo::hashAlgorithm
- Poppler::SignatureValidationInfo::signature
- Poppler::SignatureValidationInfo::signedRangeBounds
- Poppler::SignatureValidationInfo::signerSubjectDN
- Poppler::SignatureValidationInfo::signsTotalDocument

were all introduced with poppler 0.58.

Test Plan:
Builds fine with poppler 0.57 and 0.75, looked at
http://tecxoft.com/samples/pdf_digital_signature_timestamp.pdf

Reviewers: aacid

Reviewed By: aacid

Subscribers: aacid, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D20488