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

tools/lib/bpf/libbpf.c

@@ -5128,7 +5128,7 @@ int libbpf_find_vmlinux_btf_id(const char *name,
 	char *dst = raw_tp_btf + sizeof(BTF_PREFIX) - 1;
 	const char *btf_name;
 	int err = -EINVAL;
-	u32 kind;
+	__u32 kind;
 
 	if (IS_ERR(btf)) {
 		pr_warn("vmlinux BTF is not found\n");