Fixing ifndef

The call to the constructor of ShadingContext used in test_bsdfmix.cpp
is valid when both WITH_OSL and WITH_OIIO are not defined, ortherwise
it will result in a compilation problem.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Merge pull request #498 from marius-avram/oiio_ifdef_bug2

Fixing ifndef