winbuild: Update the Qt folder

Qt 5.15.0 LTS is installed on the Win10 VM. A directory symlink is made to
refer to the current version folder.

winbuild.ps1

@@ -5,7 +5,7 @@
 $OPTLIBS = "C:/opt/x64"
 $FFTW3_INCLUDE_DIR = "$OPTLIBS/include"
 $FFTW3_LIB = "$OPTLIBS/lib/libfftw3-3.lib"
-$QTDIR = "C:/Qt/5.14.1/msvc2017_64"
+$QTDIR = "C:/Qt/current/msvc2019_64"
 $QTCMake_DIR = "$QTDIR/lib/cmake"
 $BUILD_DIR = "build"