Build failure: -Wno-unused-const-variable DNE on old GCC

[Brian Norris]

Hi,

I'm using a GCC 4.6.3 compiler for some compile tests, and I noticed
that commit 2cd55c68c0a4 ("cxl: Fix build failure due to -Wunused-variable
behaviour change") breaks my builds, because the
-Wno-unused-const-variable doesn't exist on GCC 4.6.3.

  drivers/misc/cxl/base.c: At top level:
  cc1: error: unrecognized command line option "-Wno-unused-const-variable" [-Werror]

Any thoughts on how to best fix this? I'd like not to have to scrounge
up a new cross compiler just for build tests.

Regards,
Brian