Xwayland: add libei as an optional dependency

The support is added if libei is present on the system.

x/installing/xwayland.xml

@@ -89,6 +89,7 @@
     <bridgehead renderas="sect4">Optional</bridgehead>
     <para role="optional">
       <xref linkend="git"/> (to download packages needed for the tests),
+      <xref linkend="libei"/>,
       <xref linkend="libgcrypt"/>,
       <xref linkend="nettle"/>,
       <xref linkend="xmlto"/>,