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>

tools/testing/selftests/net/Makefile

@@ -3,8 +3,6 @@
 CFLAGS =  -Wall -Wl,--no-as-needed -O2 -g
 CFLAGS += -I../../../../usr/include/
 
-reuseport_bpf_numa: LDFLAGS += -lnuma
-
 TEST_PROGS := run_netsocktests run_afpackettests test_bpf.sh netdevice.sh
 TEST_GEN_FILES =  socket
 TEST_GEN_FILES += psock_fanout psock_tpacket
@@ -13,3 +11,4 @@ TEST_GEN_FILES += reuseport_dualstack
 
 include ../lib.mk
 
+$(OUTPUT)/reuseport_bpf_numa: LDFLAGS += -lnuma