perf: kill -Werror

perf: kill -Werror

this fails to build using a recent sourcery toolchain due to
unused-but-set-variable

(From OE-Core rev: dce7918a818bf86fcc11f561af3eacaf281403d4)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
the patch was imported from meta-mentor layer on yoctoproject git server
http://git.yoctoproject.org/cgit/cgit.cgi/meta-mentor as of commit ids
82e96b3baa1c64d03412871fce56d496a338f167 and
ae325d011bd50501fe677c8b37295ae83030c526

Signed-off-by: Fahad Usman <fahad_usman@mentor.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>