selftests: futex: Makefile: fix for loops in targets to run silently

Fix for loops in targets to run silently to avoid cluttering the test
results.

Suppresses the following from targets:

for DIR in functional; do               \
        BUILD_TARGET=./tools/testing/selftests/futex/$DIR; \
        mkdir $BUILD_TARGET  -p;        \
        make OUTPUT=$BUILD_TARGET -C $DIR all;\
done

./tools/testing/selftests/futex/run.sh

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Reviewed-by: Darren Hart (VMware) <dvhart@infradead.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>