Makefile.lib: align DTB quiet_cmd

The unaligned dtb.S filename in make output started to irritate me:
  DTC     arch/metag/boot/dts/skeleton.dtb
  DTB    arch/metag/boot/dts/skeleton.dtb.S
  AS      arch/metag/boot/dts/skeleton.dtb.o
  LD      arch/metag/boot/dts/built-in.o

Add an extra space to quiet_cmd_dt_S_dtb so the dtb.S filename aligns
with all the others.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Dirk Brandewie <dirk.brandewie@gmail.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: trivial@kernel.org
Signed-off-by: Michal Marek <mmarek@suse.cz>