Makefile: remove explicit libsyslog-ng-crypto references

Instead of open-coding the optional libsyslog-ng-crypto as a dependency
use $(CRYPTO_LIBS) which conditionally contains the same if
libsyslog-ng-crypto is built.

Signed-off-by: Balazs Scheidler <balazs.scheidler@balabit.com>
Merge pull request #309 from balabit/f/fix-libsyslog-ng-crypto-build-errors

Makefile: remove explicit libsyslog-ng-crypto references