libbpf: Use __u32 instead of u32 in bpf_program__load

Make bpf_program__load consistent with other interfaces: use __u32
instead of u32. That in turn fixes build of samples:

In file included from ./samples/bpf/trace_output_user.c:21:0:
./tools/lib/bpf/libbpf.h:132:9: error: unknown type name ‘u32’
         u32 kern_version);
         ^

Fixes: commit 29cd77f416 ("libbpf: Support loading individual progs")
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -1379,7 +1379,7 @@ out:
 
 int
 bpf_program__load(struct bpf_program *prog,
-		  char *license, u32 kern_version)
+		  char *license, __u32 kern_version)
 {
 	int err = 0, fd, i;