kernel-config: Explicitly enable NAMESPACES

Namespace support is required by the `unshare` tool used in
`nixos-install`. It's enabled by the x86 defconfig, but not by
e.g. multi_v7_defconfig. So enable it here so that `nixos-install`
can work on ARM.

pkgs/os-specific/linux/kernel/common-config.nix

@@ -338,6 +338,7 @@ with stdenv.lib;
   X86_MCE y
 
   # Linux containers.
+  NAMESPACES? y #  Required by 'unshare' used by 'nixos-install'
   RT_GROUP_SCHED? y
   CGROUP_DEVICE? y
   ${if versionAtLeast version "3.6" then ''