libbpf: Fix usage of u32 in userspace code

u32 is not defined for libbpf when compiled outside of kernel sources (e.g.,
in Github projection). Use __u32 instead.

Fixes: b8c54ea4 ("libbpf: Add support to attach to fentry/fexit tracing progs")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191125212948.1163343-1-andriin@fb.com