Add changes file for Qt 5.12.5

+ 7bdf0823b6 Bump version
+ d4861c2a83 Update bundled libwebp to version 1.0.3
+ 9100cc7ab0 Reapply: Fix building with clang_cl

Change-Id: Ia9b554003c3925ae570e23eff37d2ab06e4925c5
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

dist/changes-5.12.5

@@ -0,0 +1,24 @@
+Qt 5.12.5 is a bug-fix release. It maintains both forward and backward
+compatibility (source and binary) with Qt 5.12.0 through 5.12.4.
+
+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.
+
+****************************************************************************
+*                             Third-Party Code                             *
+****************************************************************************
+
+ - Update bundled libwebp to version 1.0.3