devel/zziplib: Fix build

Upstream includes COVERAGE unconditionally before this fix.
It causes build error on system without llvm-cov (WITHOUT_LLVM_COV in src.conf).

Reported by: vishwin
Obtained from: https://github.com/gdraheim/zziplib/commit/081b8ba4d0417abe6c2d997f5198e23b7fdb2977