Doxygen: exclude C++ __attribute__ (gazebosim#397)

Exclude __attribute__ keyword from C++ parsing since it is not part of the standard:
doxygen/doxygen#9142

Signed-off-by: Jose Luis Rivero <jrivero@osrfoundation.org>

doc/doxygen/api.in

@@ -2138,7 +2138,7 @@ INCLUDE_FILE_PATTERNS  =
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             =
+PREDEFINED             = __attribute__(x)=
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The