Tell Doxygen to ignore __attribute__

Doxygen can't handle it properly (see
http://doxygen.10944.n7.nabble.com/struct-attribute-packed-not-working-td2666.html)

doc/doxygen/riot.doxyfile

@@ -205,12 +205,12 @@ PERLMOD_MAKEVAR_PREFIX =
 # Configuration options related to the preprocessor   
 #---------------------------------------------------------------------------
 ENABLE_PREPROCESSING   = YES
-MACRO_EXPANSION        = NO
-EXPAND_ONLY_PREDEF     = NO
+MACRO_EXPANSION        = YES
+EXPAND_ONLY_PREDEF     = YES
 SEARCH_INCLUDES        = YES
 INCLUDE_PATH           = 
 INCLUDE_FILE_PATTERNS  = 
-PREDEFINED             = 
+PREDEFINED             =  __attribute__(x)=
 EXPAND_AS_DEFINED      = 
 SKIP_FUNCTION_MACROS   = YES
 #---------------------------------------------------------------------------