openssh: make libfido2 dependency conditional on sk-helper
The openssh-sk-helper package unconditionally depends on libfido2,
which causes the build system to pull in libfido2 and its dependencies
(libcbor, libudev) even when openssh-sk-helper is not selected.
Use the PACKAGE_openssh-sk-helper:libfido2 conditional dependency
pattern so libfido2 is only required when the sk-helper sub-package
is actually selected.