rt-tests: Add src/backfire to FILES_${PN}

rt-tests: Add src/backfire to FILES_${PN}

Fixes [YOCTO #2114]

The rt-tests recipe was issuing the following WARNINGs:

WARNING: For recipe rt-tests, the following files/directories were installed
but not shipped in any package:
WARNING:   /usr/src
WARNING:   /usr/src/backfire
WARNING:   /usr/src/backfire/backfire.c
WARNING:   /usr/src/backfire/Makefile

Add ${prefix}/src/backfire to FILES_${PN} to include these files in the package.
These depend on the target kernel and are meant to be able to be rebuilt as
needed.  Ideally we would also build this module and include it with rt-tests or
the linux-yocto-rt kernel. For now, conform with the intent of the rt-tests
Makefile and avoid the WARNINGs.

(From OE-Core rev: 04099ff7c1177b0d676591f3a5db86158738461f)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>