qbs build fix: add selectabletexteditorwidget.cpp/h files

Change-Id: Ic0c61924b3a39f6147565b31103807584e7a9598
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

src/plugins/diffeditor/diffeditor.qbs

@@ -36,6 +36,8 @@ QtcPlugin {
         "differ.h",
         "diffutils.cpp",
         "diffutils.h",
+        "selectabletexteditorwidget.cpp",
+        "selectabletexteditorwidget.h",
         "sidebysidediffeditorwidget.cpp",
         "sidebysidediffeditorwidget.h",
         "unifieddiffeditorwidget.cpp",