tools: lxc-ls: fix signed_to_bigger_unsigned issue

[2xsec]

Hello,

Fix assignment of a signed value which has type 'int' to a variable of a bigger integer type 'size_t'

Thanks.

Signed-off-by: 2xsec dh48.jeong@samsung.com

[lxc-jenkins]

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[brauner]

jenkins: test this please

[brauner]

Thanks!