graphics/libheif: fix build with clang 13

With clang 13, building this port results in a number of "variable set
but not used" warnings. These have been fixed by upstream in the mean
time.

PR: 258814
Reported by: Evgeniy Khramtsov <evgeniy@khramtsov.org>
Approved by: maintainer timeout (2 weeks)
Obtained from: https://github.com/strukturag/libheif/commit/7976e1858a857e33200cd33d01689fd040de6d3d
MFH: 2021Q4