meson.build: always set HAVE_GTK_WAYLAND
In commit 6a1bd5b0 ("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.