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 -Wpedantic -std=gnu99 -Wmissing-prototypes
-Wno-unused-parameter -Wno-shift-negative-value -Wno-missing-field-initializers -Wno-pedantic -Wundef -fvisibility=hidden -O2 -Wall -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3
-fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -g -fPIC -MD -MQ libweston/libgl-borders.a.p/gl-borders.c.o -MF
libweston/libgl-borders.a.p/gl-borders.c.o.d -o libweston/libgl-borders.a.p/gl-borders.c.o -c ../libweston/gl-borders.c
[    5s] In file included from ../libweston/renderer-gl/gl-renderer.h:32,
[    5s]                  from ../libweston/gl-borders.h:28,
[    5s]                  from ../libweston/gl-borders.c:31:
[    5s] ../include/libweston/libweston.h:39:10: fatal error: xkbcommon/xkbcommon.h: No such file or directory

[    4s] FAILED: shared/libshared.a.p/config-parser.c.o
[    4s] cc -Ishared/libshared.a.p -Ishared -I../shared -I. -I.. -Iinclude -I../include -I/usr/include/wayland -I/usr/include/pixman-1 -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall
-Winvalid-pch -Wextra -Wpedantic -std=gnu99 -Wmissing-prototypes -Wno-unused-parameter -Wno-shift-negative-value -Wno-missing-field-initializers -Wno-pedantic -Wundef -fvisibility=hidden -O2
-Wall -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -g -fPIC -MD -MQ
shared/libshared.a.p/config-parser.c.o -MF shared/libshared.a.p/config-parser.c.o.d -o shared/libshared.a.p/config-parser.c.o -c ../shared/config-parser.c
[    4s] In file included from ../shared/config-parser.c:44:
[    4s] ../include/libweston/libweston.h:39:10: fatal error: xkbcommon/xkbcommon.h: No such file or directory

Reported-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>