The libbpf/src/*.c files are included in the build system,
so those functions will be available for bcc internals to use them.

There are two name conflicts, bpf_create_map and bpf_prog_load,
between src/cc/libbpf.c and src/cc/libbpf/src/{bpf.c,libbpf.c}.
To keep src/cc/libbpf intact, the following renaming happened
in bcc repo:
  bpf_create_map  =>  bcc_create_map
  bpf_prog_load   =>  bcc_prog_load
Signed-off-by: Yonghong Song <yhs@fb.com>