Bug#708240: qtiplot FTBFS with new minizip.c

[Scott Howard]

The minizip.c that ships with the new version of zlib has changed
significantly and is no longer compatible with what qtiplot uses.

We can now just use minizip.c that comes with qtiplot

(todo: include the license in debian/copyright, drop the build
dependency on zlib, fix get-orig-source rule to not exclude it in the
future, drop part of patch that looks for minizip in working
directory)