Disable SNMP support (FTBFS due to OpenSSL transition)

libsnmp-dev depends on libssl1.0-dev, asterisk (and most of it's
dependencies) depends on libssl-dev. They cannot be coinstalled, hence
the package currently cannot be built with SNMP support

native-architecture: amd64
report:
 -
  package: sbuild-build-depends-asterisk-dummy
  version: 0.invalid.0
  architecture: amd64
  status: broken
  reasons:
   -
    conflict:
     pkg1:
      package: libssl1.0-dev
      version: 1.0.2j-5
      architecture: amd64
      unsat-conflict: libssl-dev:amd64
     pkg2:
      package: libssl-dev
      version: 1.1.0c-2
      architecture: amd64
     depchain1:
      -
       depchain:
        -
         package: sbuild-build-depends-asterisk-dummy
         version: 0.invalid.0
         architecture: amd64
         depends: libsnmp-dev:amd64
        -
         package: libsnmp-dev
         version: 5.7.3+dfsg-1.6
         architecture: amd64
         depends: libssl1.0-dev:amd64 | libssl-dev:amd64 (< 1.1)
     depchain2:
      -
       depchain:
        -
         package: sbuild-build-depends-asterisk-dummy
         version: 0.invalid.0
         architecture: amd64
         depends: libssl-dev:amd64