metag/.gitignore: Extend the *.dtb pattern to match the dtb.S files

The .SECONDARY rule for generating the *.dtb.S files added in
0b4184c2
"metag: avoid unnecessary builtin dtb rebuilds"

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: James Hogan <james.hogan@imgtec.com>