textproc/sigil: Update to 1.5.1

Changelogs: https://sigil-ebook.com/2021/03/26/sigil-1-5-1-release/,

https://github.com/Sigil-Ebook/Sigil/releases/tag/1.5.0

PR: 254618
Submitted by: Jonathan Chen <jonc@chen.org.nz> (maintainer)