pm-graph: Make git ignore sleepgraph.py artifacts

By default, sleepgraph.py creates suspend-{date}-{time} directories
to store artifacts, or suspend-{date}-{time}-xN if the --multi option
is used.

Ignore those directories by adding a .gitignore file.

Signed-off-by: Yo-Jung (Leo) Lin <0xff07@gmail.com>
Acked-by: Todd Brandt <todd.e.brandt@linux.intel.com>
Link: https://patch.msgid.link/20240825095353.7578-1-0xff07@gmail.com


[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>