tests/meson.build: Add missing dependency for xcb-client-helper

This fixes the following build issue:

[    6s] cc -Itests/libtest-xwayland-client.a.p -Itests -I../tests -I. -I.. -Iinclude -I../include -Iprotocol -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
tests/libtest-xwayland-client.a.p/xcb-client-helper.c.o -MF tests/libtest-xwayland-client.a.p/xcb-client-helper.c.o.d -o tests/libtest-xwayland-client.a.p/xcb-client-helper.c.o -c
../tests/xcb-client-helper.c
[    6s] ../tests/xcb-client-helper.c:39:10: fatal error: wayland-client.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>