qt56.qmakeHook: remove temporary directory in postInstall

Fixes #15556. If the temporary directory is not removed, it will be
retained in RPATH, which is a serious security concern.

pkgs/development/libraries/qt-5/5.6/qmake-hook.sh

@@ -74,6 +74,10 @@ _qtModuleMultioutDevsPost() {
     fi
 }
 
+_qtRmQtOut() {
+    rm -fr "$qtOut"
+}
+
 qmakeConfigurePhase() {
     runHook preConfigure
 
@@ -116,6 +120,8 @@ if [ -n "$NIX_QT_SUBMODULE" ]; then
     postInstallHooks+=(_qtRmQmake _qtRmModules)
     preFixupHooks+=(_qtModuleMultioutDevsPre)
     postFixupHooks+=(_qtModuleMultioutDevsPost)
+else
+    postInstallHooks+=(_qtRmQtOut)
 fi
 
 fi