[gn] port 92e2431

Author: nico

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

@@ -122,6 +122,7 @@ cxx_sources = [
   "condition_variable_destructor.cpp",
   "error_category.cpp",
   "exception.cpp",
+  "fstream.cpp",
   "functional.cpp",
   "future.cpp",
   "hash.cpp",