tools lib bpf: use __u32 from linux/types.h

tools lib bpf: use __u32 from linux/types.h

Fixes the following issue when building without access to 'u32' type:

./tools/lib/bpf/bpf.h:27:23: error: unknown type name ‘u32’

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Wang Nan <wangnan0@huawei.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Link: http://lkml.kernel.org/r/20161209024620.31660-3-joe@ovn.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>