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 <[e-mail]>
Acked-by: Todd Brandt <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <[e-mail]>

tools/power/pm-graph/.gitignore

···
+# sleepgraph.py artifacts
+suspend-[0-9]*-[0-9]*
+suspend-[0-9]*-[0-9]*-x[0-9]*