chore(windows): update Qt to 5.12.11

We are staying on 5.12 instead of 5.15 because 5.12 is supported for
longer. There should be one more release in H2 2021.

Ideally we would switch to 6.2, but qTox doesn't seem to be Qt6-ready
yet.

windows/cross-compile/build.sh

@@ -275,10 +275,10 @@ fi
 QT_PREFIX_DIR="$DEP_DIR/libqt5"
 QT_MAJOR=5
 QT_MINOR=12
-QT_PATCH=10
+QT_PATCH=11
 QT_VERSION=$QT_MAJOR.$QT_MINOR.$QT_PATCH
-# hash from https://download.qt.io/archive/qt/5.12/5.12.10/single/qt-everywhere-src-5.12.10.tar.xz.mirrorlist
-QT_HASH="3e0ee1e57f5cf3eeb038d0b4b22c7eb442285c62639290756b39dc93a1d0e14f"
+# hash from https://download.qt.io/archive/qt/5.12/5.12.11/single/qt-everywhere-src-5.12.11.tar.xz.mirrorlist
+QT_HASH="0c4cdef158c61827d70d6111423166e2c62b539eaf303f36ad1d0aa8af900b95"
 QT_FILENAME="qt-everywhere-src-$QT_VERSION.tar.xz"
 if [ ! -f "$QT_PREFIX_DIR/done" ]
 then