lsirq: mark --json and --pairs options mutually exclusive

lsirq: mark --json and --pairs options mutually exclusive

Before this change --pairs option would always win over --json.  That is
unnecessarily confusing, it is much better to disallow combination that does
not make sense.

Notice that the --noheadings in combination with --json or --pairs will not
cause any effect.  In strictest possible understanding --noheadings should
also be exclusive.  Looking from more relaxed point of view neither --json
nor --pairs has heading, so excluding is not necessary because --noheading
is happening already.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>