bpf: unbreak libelf linkage for bpf obj loader

bpf: unbreak libelf linkage for bpf obj loader

Commit 69fed534a533 ("change how Config is used in Makefile's") moved
HAVE_MNL specific CFLAGS/LDLIBS for building with libmnl out of the
top level Makefile into sub-Makefiles. However, it also removed the
HAVE_ELF specific CFLAGS/LDLIBS entirely, which breaks the BPF object
loader for tc and ip with "No ELF library support compiled in." despite
having libelf detected in configure script. Fix it similarly as in
69fed534a533 for HAVE_ELF.

Fixes: 69fed534a533 ("change how Config is used in Makefile's")
Reported-by: Jeffrey Panneman <jeffrey.panneman@tno.nl>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>