selftests/bpf: Restore $(OUTPUT)/test_stub.o rule

`make O=/linux-build kselftest TARGETS=bpf` fails with

	make[3]: *** No rule to make target '/linux-build/bpf/test_stub.o', needed by '/linux-build/bpf/test_verifier'

The same command without the O= part works, presumably thanks to the
implicit rule.

Fix by restoring the explicit $(OUTPUT)/test_stub.o rule.

Fixes: 74b5a5968f ("selftests/bpf: Replace test_progs and test_maps w/ general rule")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191028102110.7545-1-iii@linux.ibm.com

tools/testing/selftests/bpf/Makefile

@@ -89,6 +89,9 @@ $(notdir $(TEST_GEN_PROGS)						\
 $(OUTPUT)/urandom_read: urandom_read.c
 	$(CC) -o $@ $< -Wl,--build-id
 
+$(OUTPUT)/test_stub.o: test_stub.c
+	$(CC) -c $(CFLAGS) -o $@ $<
+
 BPFOBJ := $(OUTPUT)/libbpf.a
 
 $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED): $(OUTPUT)/test_stub.o $(BPFOBJ)