Ignore rmdir() errors

In OSX with the Xcode toolchain DWARF debug symbol files (.dSYM) will be
created when cc_command is executed (when -g is present) and the directory
will not be empty.

Prompted after b2bee0e3.

bin/varnishd/mgt/mgt_main.c

@@ -874,7 +874,7 @@ main(int argc, char * const *argv)
 		mgt_vcl_startup(cli, b_arg, f_arg, vcl, C_flag);
 		if (C_flag) {
 			if (Cn_arg == n_arg)
-				AZ(rmdir(Cn_arg));
+				(void)rmdir(Cn_arg);
 			AZ(VSB_finish(cli->sb));
 			fprintf(stderr, "%s\n", VSB_data(cli->sb));
 			exit(cli->result == CLIS_OK ? 0 : 2);