ibus: Fix cross No package 'wayland-scanner' found

The autoconf `PKG_CHECK_MODULES` macro and so on aren't made with cross
in mind, so there isn't an easy way to change them to use
PKG_CONFIG_FOR_BUILD.

`"PKG_CONFIG_WAYLAND_SCANNER_WAYLAND_SCANNER=${lib.getBin buildPackages.wayland-scanner}/bin/wayland-scanner"` in `configureFlags` isn't necessary

`wayland-scanner` is still required in `nativeBuildInputs`, otherwise
there's an error `No rule to make target '/input-method-unstable-v1.xml', needed by 'input-method-unstable-v1-protocol.c'.  Stop.`

pkgs/tools/inputmethods/ibus/default.nix

@@ -196,6 +196,7 @@ stdenv.mkDerivation (finalAttrs: {
     libxkbcommon
     wayland
     wayland-protocols
+    wayland-scanner # For cross, build uses $PKG_CONFIG to look for wayland-scanner
   ];
 
   enableParallelBuilding = true;