Fix compiler warnings on macOS

[bayandin]

Problem

Compilation of neon extension on macOS produces a warning (after #9008)

/Users/runner/work/neon/neon//pgxn/neon/neon_perf_counters.c:50:1: error: non-void function does not return a value [-Werror,-Wreturn-type]

https://github.com/neondatabase/neon/actions/runs/11015702818/job/30589419704#step:18:10437

Summary of changes

• Change return type of NeonPerfCountersShmemInit to void

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

5064 tests run: 4898 passed, 2 failed, 164 skipped (full report)

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Failures on Postgres 16

• test_storage_controller_heartbeats[failure4]: release-x86-64

Failures on Postgres 15

• test_storage_controller_heartbeats[failure4]: release-x86-64

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_storage_controller_heartbeats[release-pg15-failure4] or test_storage_controller_heartbeats[release-pg16-failure4]"

Flaky tests (8)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_cli_start_stop: debug-x86-64
• test_readonly_node_gc: release-x86-64, debug-x86-64
• test_replica_start_scan_clog_crashed_xids: release-arm64
• test_subscriber_restart: debug-x86-64

Postgres 16

• test_location_conf_churn[1]: release-arm64
• test_eager_attach_does_not_queue_up: release-x86-64

Code coverage* (full report)

• functions: 32.1% (7471 of 23243 functions)
• lines: 50.0% (60201 of 120505 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
4677a95 at 2024-09-24T18:22:07.708Z :recycle:}