selftest/net/Makefile: Specify output with $(OUTPUT)

After commit a8ba798b ("selftests: enable O and KBUILD_OUTPUT"),
net selftest build fails because it points output file without $(OUTPUT)
yet.  This commit fixes the error.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Fixes: a8ba798b ("selftests: enable O and KBUILD_OUTPUT")
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>