[gn build] Port dedc515

Author: llvmgnsyncbot

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

@@ -790,6 +790,7 @@ if (current_toolchain == default_toolchain) {
       "__system_error/error_code.h",
       "__system_error/error_condition.h",
       "__system_error/system_error.h",
+      "__system_error/throw_system_error.h",
       "__thread/formatter.h",
       "__thread/id.h",
       "__thread/jthread.h",