Clang Wself-assign warning during build process on some platforms

[Nicolas Martignoni]

During build on Mac OS X and other platforms, some Wself-assign warnings are raised by code, as seen in Buildbot (http://gcc10.fsffrance.org:8010/waterfall).

Files affected:
plugins/Satellites/src/gsatellite/sgp4unit.cpp
src/core/external/kfilter/kzip.cpp

Patch to fix warnings attached, using
  static_cast<void>(var);
instead of
  var = var;

[Alexander Wolf (alexwolf)]

Diff: http://bazaar.launchpad.net/~stellarium/stellarium/trunk/revision/5750