Fix: lay out names in exclusion structure correctly

This:

    char names[0][LTTNG_SYMBOL_NAME_LEN];

means that accessing

    ->names[0][i]

will return the i-th character of the first name.
So doing:

    lttng enable-event -u -a --exclude hello,a,b,c,d
    lttng save

would result in a saved session XML file containing
this:

    <exclusion>hello</exclusion>
    <exclusion>ello</exclusion>
    <exclusion>llo</exclusion>
    <exclusion>lo</exclusion>
    <exclusion>o</exclusion>

This is great art, but not what is expected.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>