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