kdeFrameworks.kpurpose: add patch required with Qt5 5.15.2

Qt 5.15 carried a change that renamed a getter for the socket error. In
a later relase that change was reverted (to be less ambiguous) thus now
 those that had picked up the change in API had to change their code.
 This has now been reverted [0] and thus the consumers also have to
 revert their changes…

[0] https://codereview.qt-project.org/c/qt/qtbase/+/289693

pkgs/development/libraries/kde-frameworks/purpose.nix

@@ -1,12 +1,19 @@
 {
   mkDerivation, lib, extra-cmake-modules, qtbase
 , qtdeclarative, kconfig, kcoreaddons, ki18n, kio, kirigami2
+, fetchpatch
 }:
 
 mkDerivation {
   name = "purpose";
   meta = { maintainers = [ lib.maintainers.bkchr ]; };
   nativeBuildInputs = [ extra-cmake-modules ];
+  patches = [
+    (fetchpatch {
+      url = "https://github.com/KDE/purpose/commit/b3842a0941858792e997bb35b679a3fdf3ef54ca.patch";
+      sha256 = "14brpryrrfrli1amk4flpnd03wr4zyycpiirndn9sjz0krqlgf3j";
+    })
+  ];
   buildInputs = [
     qtbase qtdeclarative kconfig kcoreaddons
     ki18n kio kirigami2