SNMP: Fix warning found by -Wunused-const-variable

Change-Id: If3fcab128e3e6bce04f4922568ab7ff8575b01fe
warning: unused variable 'smux_types' [-Wunused-const-variable]
Reviewed-on: https://code.wireshark.org/review/5194
Reviewed-by: Evan Huus <eapache@gmail.com>