Add changes file for Qt 5.12.6

+ 05ad406b Update the requirements doc for Linux to reflect XCB 1.9 required
+ 20760875 List xkbcommon in Qt for X11 Requirements
+ 143e36e0 Remove 'Open Source Licensing of Qt' page
+ d29777f6

 Bump version

Change-Id: I1eee44bca4eb74fe7c0f9b85a4ba4bb3d0633311
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

dist/changes-5.12.6

+Qt 5.12.6 is a bug-fix release. It maintains both forward and backward
+compatibility (source and binary) with Qt 5.12.0 through 5.12.5.
+
+For more details, refer to the online documentation included in this
+distribution. The documentation is also available online:
+
+https://doc.qt.io/qt-5/index.html
+
+The Qt version 5.12 series is binary compatible with the 5.11.x series.
+Applications compiled for 5.11 will continue to run with 5.12.
+
+Some of the changes listed in this file include issue tracking numbers
+corresponding to tasks in the Qt Bug Tracker:
+
+https://bugreports.qt.io/
+
+Each of these identifiers can be entered in the bug tracker to obtain more
+information about a particular change.
+
+ - This release contains only minor code improvements.