busybox: make busybox's syslog.cfg depend on VIRTUAL-RUNTIME_base-utils-syslog

syslog.cfg is added to the list of sources for busybox
independent of the VIRTUAL-RUNTIME_base-utils-syslog variable. So even
if VIRTUAL-RUNTIME_base-utils-syslog being set e.g. to empty, syslogd will
be enabled. So only include syslog.cfg in SRC_URI if
VIRTUAL-RUNTIME_base-utils-syslog is set to busybox-syslog.

Signed-off-by: Volker Vogelhuber <v.vogelhuber@digitalendoscopy.de>
Signed-off-by: Steve Sakoman <steve@sakoman.com>