gnu: libxkbcommon: Fix cross-compiling.

* gnu/packages/xdisorg.scm (libxkbcommon)[native-inputs]: When
cross-compiling add pkg-config-for-build.

gnu/packages/xdisorg.scm

@@ -527,9 +527,13 @@ avoiding password prompts when X11 forwarding has already been setup.")
            wayland-protocols
            xkeyboard-config))
     (native-inputs
+     (append
        (list bison doxygen pkg-config python
              ;; wayland-scanner is required at build time.
-           wayland))
+             wayland)
+       (if (%current-target-system)
+         (list pkg-config-for-build)
+         '())))
     (arguments
      (list #:configure-flags
            #~(list (string-append "-Dxkb-config-root="