add kernel libbpf as a submodule

The kernel libbpf library
  https://github.com/torvalds/linux/tree/master/tools/lib/bpf
provides common functionality for handling bpf programs, maps,
ELF parsing, probing, BTF manipulation, etc. The library has
been recommended to the community as a common library for
bpf/btf handling. A separate repository
  https://github.com/libbpf/libbpf
has been created for this purpose.

This patch brings in libbpf to bcc. The following command
is executed to crate a libbpf submodule at src/cc/libbpf:
  git submodule add https://github.com/libbpf/libbpf.git src/cc/libbpf

Signed-off-by: Yonghong Song <[email protected]>

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "src/cc/libbpf"]
+	path = src/cc/libbpf
+	url = https://github.com/libbpf/libbpf.git