libbpf: Fix build errors.

These elf object pieces are of type Elf64_Xword and therefore could be
"long long" on some builds.

Cast to "long long" and use printf format %lld to deal with this since
we are building with -Werror=format.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>