selftests/net/tcp-ao: Use LDLIBS instead of LDFLAGS

selftests/net/tcp-ao: Use LDLIBS instead of LDFLAGS

The rules to link selftests are:

> $(OUTPUT)/%_ipv4: %.c
> 	$(LINK.c) $^ $(LDLIBS) -o $@
>
> $(OUTPUT)/%_ipv6: %.c
> 	$(LINK.c) -DIPV6_TEST $^ $(LDLIBS) -o $@

The intel test robot uses only selftest's Makefile, not the top linux
Makefile:

> make W=1 O=/tmp/kselftest -C tools/testing/selftests

So, $(LINK.c) is determined by environment, rather than by kernel
Makefiles. On my machine (as well as other people that ran tcp-ao
selftests) GNU/Make implicit definition does use $(LDFLAGS):

> [dima@Mindolluin ~]$ make -p -f/dev/null | grep '^LINK.c\>'
> make: *** No targets.  Stop.
> LINK.c = $(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) $(TARGET_ARCH)

But, according to build robot report, it's not the case for them.
While I could just avoid using pre-defined $(LINK.c), it's also used by
selftests/lib.mk by default.

Anyways, according to GNU/Make documentation [1], I should have used
$(LDLIBS) instead of $(LDFLAGS) in the first place, so let's just do it:

> LDFLAGS
>     Extra flags to give to compilers when they are supposed to invoke
>     the linker, ‘ld’, such as -L. Libraries (-lfoo) should be added
>     to the LDLIBS variable instead.
> LDLIBS
>     Library flags or names given to compilers when they are supposed
>     to invoke the linker, ‘ld’. LOADLIBES is a deprecated (but still
>     supported) alternative to LDLIBS. Non-library linker flags, such
>     as -L, should go in the LDFLAGS variable.

[1]: https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html

Fixes: cfbab37b3da0 ("selftests/net: Add TCP-AO library")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401011151.veyYTJzq-lkp@intel.com/
Signed-off-by: Dmitry Safonov <dima@arista.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240110-tcp_ao-selftests-makefile-v1-1-aa07d043f052@arista.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>