Forward-declare LayoutUnit consistently (as class, not struct)

Resolves issues like this one:

src/kde/plasma/plasma-desktop/kcms/keyboard/flags.h:29:1: warning:
      struct 'LayoutUnit' was previously declared as a struct; this is valid,
      but may result in linker errors under the Microsoft C++ ABI
      [-Wmismatched-tags]
struct LayoutUnit;
^
src/kde/plasma/plasma-desktop/kcms/keyboard/x11_helper.h:90:7: note:
      previous use is here
class LayoutUnit {