Add missing file to BUILD.gn (#3798)

Fixes #3797.
diff --git a/BUILD.gn b/BUILD.gn
index 995e172..de4e48e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -774,6 +774,7 @@
     "source/reduce/reducer.h",
     "source/reduce/reduction_opportunity.cpp",
     "source/reduce/reduction_opportunity.h",
+    "source/reduce/reduction_opportunity_finder.cpp",
     "source/reduce/reduction_opportunity_finder.h",
     "source/reduce/reduction_pass.cpp",
     "source/reduce/reduction_pass.h",