ptpd2: handle snmp optional dep

Handle the optional net-snmp dep or be explicit in disabling it since
since it can pick up a distribution net-snmp-config and pollute
everything. Hopefully fixes:
http://autobuild.buildroot.net/results/5bd/5bdfcf544a83e18d12e27c598084c0ad520d6164/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>