openssl: fix variable reference in conffiles

openssl: fix variable reference in conffiles

Fix the trivial abscence of $() when assigning engine config files to
the main libopenssl-config package even if the corresponding engines
were not built into the main library.

This is mostly cosmetic, since scripts/ipkg-build tests the file's
presence before it is actually included in the package's conffiles.

Fixes: 30b0351039 "openssl: configure engine packages during install"
Signed-off-by: Eneas U de Queiroz <[email protected]>