kbuild: use $(Q) for sub-make target

Since commit 066b7ed9
(kbuild: Do not print the build directory with make -s),
"Q" is defined above the sub-make target.

This commit takes advantage of that and replaces
"$(if $(KBUILD_VERBOSE:1=),@)" with "$(Q)".

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>