libbpf: Ignore -Wformat-nonliteral warning

vsprintf() in __base_pr() uses nonliteral format string and it breaks
compilation for those who provide corresponding extra CFLAGS, e.g.:
https://github.com/libbpf/libbpf/issues/27



If libbpf is built with the flags from PR:

  libbpf.c:68:26: error: format string is not a string literal
  [-Werror,-Wformat-nonliteral]
          return vfprintf(stderr, format, args);
                                  ^~~~~~
  1 error generated.

Ignore this warning since the use case in libbpf.c is legit.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>