gdb: Fix "scylla column_families" command
gdb: Fix "scylla column_families" command

Apparently some GDB versions (7.11.1-86.fc24) don't parse double '>'
in a type name, so this:

 std::pair<utils::UUID const, seastar::lw_shared_ptr<column_family>>

should be this:

 std::pair<utils::UUID const, seastar::lw_shared_ptr<column_family> >

Message-Id: <5302332bd08b2da08eff16c65b68906fd8ee7a19@scylladb.com>

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