selftests/bpf: Fix LDLIBS order

Order of $(LDLIBS) matters to linker, so put it after all the .o and .a
files.

Fixes: 74b5a596 ("selftests/bpf: Replace test_progs and test_maps w/ general rule")
Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191023153128.3486140-1-andriin@fb.com