update_headers.sh: Add missing shell quoting

update_headers.sh can break if the current working directory has a
funny name or if something odd is passed for LINUX_ROOT.

In the interest of cleanliness, quote where appropriate.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>