lxc-clone: proper type for getopt_long result

lxc-clone: proper type for getopt_long result

While char is a signed type and should have no trouble
storing a "-1" value, getopt_long() will return a value of 255
instead of -1. This has been noticed on powerpc; there's also
some upstream talk about it at [1].

Change variable type from char to int, since it's also the
documented use.

[1] http://stackoverflow.com/questions/17070958/c-why-does-getopt-return-255-on-linux

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>