tools: bpftool: silence 'missing initializer' warnings

When building bpf tool, gcc emits piles of warnings:

prog.c: In function ‘prog_fd_by_tag’:
prog.c:101:9: warning: missing initializer for field ‘type’ of ‘struct bpf_prog_info’ [-Wmissing-field-initializers]
  struct bpf_prog_info info = {};
         ^
In file included from /home/storage/jbenc/git/net-next/tools/lib/bpf/bpf.h:26:0,
                 from prog.c:47:
/home/storage/jbenc/git/net-next/tools/include/uapi/linux/bpf.h:925:8: note: ‘type’ declared here
  __u32 type;
        ^

As these warnings are not useful, switch them off.

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