Bug#1026441: snmpkit: FTBFS due to ISO C++17 standard

[Eric Long]

According to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=984286, 
maybe fix those `throw` statements to comply C++17 standard:

> GCC 11 defaults to the GNU++17 standard. If your package installs
> header files in /usr/include, please don't work around C++17 issues
> by choosing a lower C++ standard for the package build, but fix these
> issues to build with the C++17 standard.
Hope this link will help.

Cheers,
Eric