fix generation of OpaqueFwd in presence of user-written opaque convertion functions

The actual opaques that the user specifies were not added to the Fwd
header, which meant that in principle the compilation would fail.

We've been lucky so far it seems.
Merge pull request #17 from orocos-toolchain/fix_opaquefwd_generation

fix generation of OpaqueFwd in presence of user-written opaque convertion functions