Cleanup: Move opensubdiv C-API headers to C++

All the relevant code is C++ now, so we don't need to complicate things
with the trip through C anymore. We will still need some wrappers, since
opensubdiv is an optional dependency though. The goal is to make it
simpler to remove the unnecessary/costly abstraction levels between
Blender mesh data and the opensubdiv code.

source/blender/python/intern/bpy_app_opensubdiv.cc

@@ -14,7 +14,7 @@
 #include "../generic/py_capi_utils.h"
 
 #ifdef WITH_OPENSUBDIV
-#  include "opensubdiv_capi.h"
+#  include "opensubdiv_capi.hh"
 #endif
 
 static PyTypeObject BlenderAppOpenSubdivType;