meson.build: always set HAVE_GTK_WAYLAND

meson.build: always set HAVE_GTK_WAYLAND

In commit 6a1bd5b0c9be ("meson.build: check gtk targets before
building") introduced a custom config option to check whether Wayland
is supported by GTK or not.

However, in some cases the config option is not set generating this
warning:

    ../tools/libinput-debug-gui.c:51:5: warning: "HAVE_GTK_WAYLAND" is
    not defined, evaluates to 0 [-Wundef]
       51 | #if HAVE_GTK_WAYLAND
          |     ^~~~~~~~~~~~~~~~

Make sure to always set HAVE_GTK_WAYLAND.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>