Fix compilation error with latest OIIO 1.7.8

There are some changes in OIIO includes so now need to do some
things differently.

source/blender/imbuf/intern/oiio/CMakeLists.txt

@@ -49,6 +49,11 @@ if(WITH_OPENIMAGEIO)
 		${OPENIMAGEIO_INCLUDE_DIRS}
 		${BOOST_INCLUDE_DIR}
 	)
+	if(WITH_IMAGE_OPENEXR)
+		list(APPEND INC_SYS
+			${OPENEXR_INCLUDE_DIRS}
+		)
+	endif()
 	add_definitions(-DWITH_OPENIMAGEIO)
 endif()