libweston,shared/meson:build Add xkbcommon missing depends

This is because e619a65b, 'libweston: move gl-borders code into
helper lib' and 6293ab1f, 'libweston, shared: Move out
weston_shell_get_binding_modifier' moved things out of libweston, and
libweston implicitly depends on xkbcommon.

Rather than just depending on dep_xkbcommon use the deps_for_libweston_users
which includes some other dependencies as well. Had to move it out
of libweston/meson.build and include it in the main meson.build as
libweston/meson.build would have a circular dependency on
libweston/meson.build file.

This fixes the following build issue:

[    5s] FAILED: libweston/libgl-borders.a.p/gl-borders.c.o
[    5s] cc -Ilibweston/libgl-borders.a.p -Ilibweston -I../libweston -I. -I.. -Iinclude -I../include -I/usr/include/wayland -I/usr/include/pixman-1 -I/usr/include/cairo
-I/usr/include/libpng16 -I/usr/include/freetype2 -I/usr/include/webp -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -Wpeda...