kbuild: silence generated makefile message

This patch silences the "make -C /usr/src/git O=/usr/src/git/build/."
message shown when using the generated makefile in KBUILD_OUTDIR.

Signed-off-by: Peter Foley <pefoley2@verizon.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>