windows.cfg: Add _countof macro configuration (#1047)

By telling cppcheck about the Microsoft Windows _countof() macro it is
now able to detect out of bounds array accesses like this:
wchar_t buf[10];
for(size_t i = 0; i <= (_countof(buf)); ++i) {
buf[i] = L'\0'; // (error) Array 'buf[10]' accessed at index 10, which
is out of bounds.
}

cfg/windows.cfg

@@ -4060,4 +4060,5 @@ HFONT CreateFont(
   <define name="WINAPI" value=""/>
   <define name="__pragma(x)" value=""/>
   <define name="TEXT(str)" value="str"/>
+  <define name="_countof(_Array)" value="(sizeof(_Array) / sizeof(_Array[0]))"/>
 </def>