problem.hh: fix BOOST_MPL_ASSERT_MSG error for abstract types.

While compiling the Ipopt plugin, the following compiler error appeared
in BOOST_MPL_ASSERT_MSG:

  error: invalid abstract parameter type ‘roboptim::GenericDifferentiableFunction<roboptim::EigenMatrixDense>’

This was fixed with an idea coming from:
https://svn.boost.org/trac/boost/ticket/1814