[common]: move array length into a common helper

Since it is more generally useful, and less cryptic this way.

common/include/common/array.h

@@ -0,0 +1,26 @@
+/**
+ * Looking Glass
+ * Copyright (C) 2017-2021 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#ifndef _LG_ARRAY_H_
+#define _LG_ARRAY_H_
+
+#define ARRAY_LENGTH(arr) (sizeof(arr) / sizeof(*(arr)))
+
+#endif