[gn] port 03c19e9

Author: nico

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

@@ -643,6 +643,7 @@ if (current_toolchain == default_toolchain) {
       "__numeric/pstl_reduce.h",
       "__numeric/pstl_transform_reduce.h",
       "__numeric/reduce.h",
+      "__numeric/saturation_arithmetic.h",
       "__numeric/transform_exclusive_scan.h",
       "__numeric/transform_inclusive_scan.h",
       "__numeric/transform_reduce.h",