gdb: Fix --size option of "scylla task_histogram"
gdb: Fix --size option of "scylla task_histogram"

By default, argparse will provide the value of the option as str.
Later, we compare it with int, which will be always False. Fix by
telling argparse to provide as int.
Message-Id: <0ebc6ca103bd804b44fc8b09359fc988ac260acb@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla