support/scripts/check-bin-arch: ignore /usr/lib/grub

/lib/grub is already ignored, so add /usr/lib/grub to support
BR2_ROOTFS_MERGED_USR.

Signed-off-by: Alex Xu <alex_y_xu@yahoo.ca>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>