MinGW plugins should not have the lib prefix in the shared library
names. Do this manually for a couple of libraries, and also for the
generic qt_add_cmake_library function.
Amends 9b0e23ef8a
Change-Id: I1cfaf8fc046f86edd3e755adfa599aa0aa854ee3
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>