[gn build] (manually) port 157bbe6

Author: nico

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

@@ -336,7 +336,6 @@ if (current_toolchain == default_toolchain) {
       "__memory/voidify.h",
       "__mutex_base",
       "__node_handle",
-      "__nullptr",
       "__numeric/accumulate.h",
       "__numeric/adjacent_difference.h",
       "__numeric/exclusive_scan.h",