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