Fixes cmake policy CMP0028 (Qt5::OpenGL missing)

with cmake 3.0.2

CMake Warning (dev) at source/gloperate-qtwidgets/CMakeLists.txt:97 (add_library):
  Policy CMP0028 is not set: Double colon in target name means ALIAS or
  IMPORTED target.  Run "cmake --help-policy CMP0028" for policy details.
  Use the cmake_policy command to set the policy and suppress this warning.

  Target "gloperate-qtwidgets" links to target "Qt5::OpenGL" but the target
  was not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?
This warning is for project developers.  Use -Wno-dev to suppress it.