package/syslog-ng: fix build if net-snmp is installed on the host (again)

package/syslog-ng: fix build if net-snmp is installed on the host (again)

See also commit 4ff6e5239240a4e1bdf73c41d6e9bff9473ec62c which describes the
problem in detail.

The same problem now arises again, because syslog-ng renamed the
--enable-snmp-dest option into --enable-afsnmp. See syslog-ng commit
4537938474771673ef5bd4a9cad7c9a7dc20b7c1, first part of 3.27.1.

Update the configure options passed from Buildroot accordingly.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>