libbpf: Compile using -std=gnu89

The minimum supported C standard version is C89, with use of GNU
extensions, hence make sure to catch any instances that would break
the build for this mode by passing -std=gnu89.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211105234243.390179-4-memxor@gmail.com