metag/.gitignore: Extend the *.dtb pattern to match the dtb.S files
The .SECONDARY rule for generating the *.dtb.S files added in
0b4184c26b18ecbc0ec0657f514b087c3a6216fb
"metag: avoid unnecessary builtin dtb rebuilds"
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Markos Chandras