tools: bpftool: fix compilation with older headers

Compilation of bpftool on a distro that lacks eBPF support in the installed
kernel headers fails with:

common.c: In function ‘is_bpffs’:
common.c:96:40: error: ‘BPF_FS_MAGIC’ undeclared (first use in this function)
  return (unsigned long)st_fs.f_type == BPF_FS_MAGIC;
                                        ^
Fix this the same way it is already in tools/lib/bpf/libbpf.c and
tools/lib/api/fs/fs.c.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>