List xkbcommon-x11 in Qt for X11 Requirements

Fixes: QTBUG-75523
Change-Id: I3954cfedaa8ad93971162c7293a22ec5510fd625
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

doc/src/platforms/linux.qdoc

@@ -211,6 +211,12 @@
     \li Keymap handling
     \li \tt{-xkbcommon} or auto-detected
     \li 0.5.0
+    \row {id="OptionalColor"}
+    \li xkbcommon-x11
+    \li libxkbcommon-x11
+    \li Keymap handling
+    \li auto-detected
+    \li 0.5.0
 
     \row {id="OptionalColor"}
     \li Fontconfig