perf bpf: Provide a weak btf__load_from_kernel_by_id() for older libbpf versions

The btf__get_from_id() function was deprecated in favour of
btf__load_from_kernel_by_id(), but it is still avaiable, so use it to
provide a weak function btf__load_from_kernel_by_id() for older libbpf
when building perf with LIBBPF_DYNAMIC=1, i.e. using the system's libbpf
package.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>