Doxygen: define DILIGENT_CPP_INTERFACE macros

Author: TheMostDiligent

Doc/doxygen.cfg

@@ -2547,7 +2547,8 @@ PREDEFINED             = __cplusplus
 # definition found in the source code.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-EXPAND_AS_DEFINED      = DILIGENT_BEGIN_NAMESPACE \
+EXPAND_AS_DEFINED      = DILIGENT_CPP_INTERFACE \
+                         DILIGENT_BEGIN_NAMESPACE \
                          DILIGENT_END_NAMESPACE \
                          DILIGENT_TYPED_ENUM \
                          DILIGENT_DERIVE \