List xkbcommon in Qt for X11 Requirements

Fixes: QTBUG-77919
Change-Id: Id01fba9954cf8d86d813e1f1c270cb5c7f62c8d2
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

doc/src/platforms/linux.qdoc

@@ -205,6 +205,13 @@
     \li auto-detected or provided by -qt-xcb
     \li 0.3.9
 
+    \row {id="OptionalColor"}
+    \li xkbcommon
+    \li libxkbcommon
+    \li Keymap handling
+    \li \tt{-xkbcommon} or auto-detected
+    \li 0.5.0
+
     \row {id="OptionalColor"}
     \li Fontconfig
     \li libfontconfig