contrib: qt: strip installed static libraries with gcc+win32

Steve Lhommerequested to merge
robUx4/vlc:qt-gcc-strip into master

Since the generated DLL is too big if it contains the symbols (#28643, #28598 (closed)) we strip the symbols. Since it won't be used to make release builds, it doesn't matter.

version 5: rebased and remove the patch about QT_HOST_PATH not needed anymore

Edited by Steve Lhomme

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