meson.build: manually link all sources into the library

meson.build: manually link all sources into the library

This works around meson bug 3937, 'link_whole' arguments don't get added into
the final static library and we end up with a virtually empty 8-byte
libxkbcommon.a file, see https://github.com/mesonbuild/meson/issues/3937

Workaround is simply to add all sources to both libraries we need them in.
This obviously compiles them twice but this year's winter was cold and
bit of extra warmth will be appreciated.

Fixes #84

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>