powerpc: Ignore generated vmlinux.lds in git

Add a .gitignore in arch/powerpc/kernel to ignore the generated
vmlinux.lds.
Signed-off-by: NSebastien Dugue <sebastien.dugue@bull.net>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/.gitignore

+vmlinux.lds