Add the new evergreen/system_perf/master/compiles_pgo.yml DSI fragment to the sys-perf project's include list in etc/system_perf.yml.

This is the mongo-repo companion to DEVPROD-36738, which splits the four Stage-2/Stage-3 LTO PGO BOLT compile variants out of DSI's compiles.yml into a new compiles_pgo.yml so that compiles.yml can be included on its own by config files that do not include variants.yml (needed by the dedicated sys-perf-soak project config).

The sys-perf variant set is unchanged: 212 build variants before and after, verified via evergreen evaluate. This must land together with the DSI compiles_pgo.yml changes.

Additional Context

• DEVPROD-36738 Split PGO/BOLT compile variants into compiles_pgo.yml_: the DSI-side change this PR accompanies; DSI PRs 10gen/dsi#5288 (empty placeholder) and 10gen/dsi#5287 (variant move).