perf llvm: Add debug hint message about missing kernel-devel package

To help in debugging, add this extra message:

detect_kbuild_dir: Couldn't find "/lib/modules/5.4.20-200.fc31.x86_64/build/include/generated/autoconf.h", missing kernel-devel package?.

Cc: Adrian Hunter <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/llvm-utils.c

···
 			return -ENOMEM;
 		return 0;
 	}
+	pr_debug("%s: Couldn't find \"%s\", missing kernel-devel package?.\n",
+		 __func__, autoconf_path);
 	free(autoconf_path);
 	return -ENOENT;
 }