wintab32: Use the ARRAY_SIZE() macro.

dlls/wintab32/wintab32.c

@@ -89,7 +89,7 @@ static HMODULE load_graphics_driver(void)
     HKEY hkey;
     DWORD size;
     WCHAR path[MAX_PATH];
-    WCHAR key[(sizeof(key_pathW) + sizeof(displayW)) / sizeof(WCHAR) + 40];
+    WCHAR key[ARRAY_SIZE(key_pathW) + ARRAY_SIZE(displayW) + 40];
     UINT guid_atom = HandleToULong( GetPropW( GetDesktopWindow(), display_device_guid_propW ));
 
     if (!guid_atom) return 0;