snmppp: bump to version 3.3.4

Changes snmp++v3.3.4:
- Fixed: Incorrect time values conversion in += and -= operators of the
  msec class in msec.cpp.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>