Changed formula for getting the number of elements in a static array

When using the formula [sizeof(array) / sizeof( x )] to get the number
of element in a static array, it is better to use array[0] for 'x'
instead of the base type of array:
 - in case the base type would change someday;
 - if the compiler were deciding to insert padding somewhere

src/wmspec.c

@@ -202,7 +202,7 @@ static atomitem_t atomNames[] = {
 	{"UTF8_STRING", &utf8_string},
 };
 
-#define atomNr (sizeof(atomNames)/sizeof(atomitem_t))
+#define atomNr (sizeof(atomNames)/sizeof(atomNames[0]))
 
 #define _NET_WM_STATE_REMOVE 0
 #define _NET_WM_STATE_ADD 1