libbpf: Fix Makefile' libbpf symbol mismatch diagnostic

Fix Makefile's diagnostic diff output when there is LIBBPF_API-versioned
symbols mismatch.

Fixes: 1bd63524

 ("libbpf: handle symbol versioning properly for libbpf.a")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191127200134.1360660-1-andriin@fb.com