[gn] port cf7d4f5

Author: nico

llvm/utils/gn/secondary/libcxx/src/BUILD.gn

@@ -115,6 +115,7 @@ cxx_sources = [
   "atomic.cpp",
   "barrier.cpp",
   "bind.cpp",
+  "call_once.cpp",
   "charconv.cpp",
   "chrono.cpp",
   "condition_variable.cpp",