xkeyboard-config: make available outside of X.org, reduce dependencies

xkeyboard-connfig was selectable only if X.org was enabled. However,
weston, the reference implementation of the Wayland protocol, also
needs xkeyboard-config, so we have to make this package available
outside of the if BR2_PACKAGE_XORG7 ... endif conditional.

In addition to this, the xkeyboard-config currently pulls in
xapp_xkbcomp as a runtime dependency, but this dependency is only
needed with X.org. And it also pulls in xlib_libX11 and xproto_proto
as build-time dependencies. But in fact those ones are runtime
dependencies, and they are only needed under X.org. This helps
reducing the number of dependencies of xkeyboard-config in a
weston/wayland configuration.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>