selftests: powerpc: Fix warning for security subdir

typing 'make' inside tools/testing/selftests/powerpc gave a build
warning:

BUILD_TARGET=tools/testing/selftests/powerpc/security; mkdir -p $BUILD_TARGET; make OUTPUT=$BUILD_TARGET -k -C security all
make[1]: Entering directory 'tools/testing/selftests/powerpc/security'
../../lib.mk:20: ../../../../scripts/subarch.include: No such file or directory
make[1]: *** No rule to make target '../../../../scripts/subarch.include'.
make[1]: Failed to remake makefile '../../../../scripts/subarch.include'.

The build is one level deeper than lib.mk thinks it is. Set top_srcdir
to set things straight.

Note that the test program is still built.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>