fix: _compile_tree to use depends_on for rtree.c instead of source_files

[tlambert03]

fixing the broken tests

SRC_DIR / "src" / "rtree.c"

is #included and doesn't need to be listed as a compilation/link target.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.11%. Comparing base (32de8f7) to head (a048012).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #54   +/-   ##
=======================================
  Coverage   95.11%   95.11%           
=======================================
  Files          12       12           
  Lines         389      389           
=======================================
  Hits          370      370           
  Misses         19       19           

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[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

---

_{Comparing tlambert03:fix-rtree (a048012) with main (32de8f7)¹}

Footnotes

1. No successful run was found on main (ddf69f3) during the generation of this report, so 32de8f7 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩