Rename libclipper namespace

So it won't clash with the libclipper used by the Mapbox GL plugin
when linking Qt with -static.

With -static, all the plugins are linked together instead of each
one having its own shared library, thus, hiding private symbols.

Change-Id: I33ee4afd4b83741c5f3fae32a88da7f4a7fee75f
Reviewed-by: Paolo Angelelli <paolo.angelelli@qt.io>