api: Restore OIIO_NAMESPACE_USING (#4603)

This got lost with the 2-level namespace changes in #4567, but is used
by e.g. OpenUSD. Might as well not break the API here.

Signed-off-by: Brecht Van Lommel <brecht@blender.org>

src/include/OpenImageIO/oiioversion.h.in

@@ -178,6 +178,9 @@ namespace OIIO = @PROJ_NAMESPACE@;
 #define OIIO_NAMESPACE_3_0_BEGIN namespace OIIO_NAMESPACE_3_0 {
 #define OIIO_NAMESPACE_3_0_END }
 
+// Macro to use names without explicit qualifier
+#define OIIO_NAMESPACE_USING using namespace @PROJ_NAMESPACE@::@PROJ_VERSION_NAMESPACE@;
+
 
 /// Each imageio DSO/DLL should include this statement:
 ///      DLLPUBLIC int FORMAT_imageio_version = OPENIMAGEIO_PLUGIN_VERSION;