[Patch, MIPS] Fix unused function warning

[Joseph Myers]

On Wed, 10 Dec 2014, Steve Ellcey  wrote:

> 2014-12-10  Steve Ellcey  <sellcey@imgtec.com>
> 
> 	* debug/warning-nop.c: Add used atrribute.

OK.

(I imagine this is actually compiler-version-dependent, not 
architecture-dependent.)

-- 
Joseph S. Myers
joseph@codesourcery.com