Don't include xkbcommon/xkbcommon.h in input.h, forward-declare instead

Some systems (e.g. openSuSE) don't install the xkbcommon header into
/usr/include/xkbcommon/xkbcommon.h (which is always in the include path),
but instead into a subdirectory, which is in the openSuSE case
/usr/include/pkg/libxkbcommon/xkbcommon/xkbcommon.h. This means that e.g.
kcm_kwinrules will not compile there since it includes input.h

REVIEW: 117069