build: make xkbcommon built-time dependency

build: make xkbcommon built-time dependency

We really need xkbcommon-keysyms.h for building kmscon/uterm/TSM/etc.
However, the recent fix was ugly and didn't really help. Instead we copy
the keysyms file into external/xkbcommon/ so we can just include the real
xkbcommon files from any source but have a fallback in external/.

Hence, you can still build kmscon without xkbcommon with this fallback,
but this will be removed the first day when xkbcommon sees a public
release.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>