bpftool: Simplify format string to not use positional args

Change format string referring to just single argument out of two available.
Some versions of libc can reject such format string.
Reported-by: Nikita Shirokov <tehnerd@tehnerd.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191218214314.2403729-1-andriin@fb.com

tools/bpf/bpftool/gen.c

@@ -567,9 +567,9 @@ static int do_skeleton(int argc, char **argv)
 			return -1;					    \n\
 		}							    \n\
 									    \n\
-		#endif /* %2$s */					    \n\
+		#endif /* %s */						    \n\
 		",
-		obj_name, header_guard);
+		header_guard);
 	err = 0;
 out:
 	bpf_object__close(obj);