net-snmp: fix failure when starting on system without pci bus

The snmpd daemon cannot start on platforms without a pci bus:

snmpd[15460]: pcilib: Cannot open /proc/bus/pci
snmpd[15460]: pcilib: Cannot find any working access method.

snmpd should not depend on a PCI bus and should simply move on.

Signed-off-by: Jack Mitchell <jack.mitchell@dbbroadcast.co.uk>