doc/doxygen.conf: generate documentation for C++ headers

doc/doxygen.conf.in

@@ -538,7 +538,7 @@ INPUT_ENCODING         = UTF-8
 # *.c *.cc *.cxx *.cpp *.c++ *.java *.ii *.ixx *.ipp *.i++ *.inl *.h *.hh *.hxx
 # *.hpp *.h++ *.idl *.odl *.cs *.php *.php3 *.inc *.m *.mm *.py *.f90
 
-FILE_PATTERNS          = *.h
+FILE_PATTERNS          = *.h *.hxx
 
 # The RECURSIVE tag can be used to turn specify whether or not subdirectories
 # should be searched for input files as well. Possible values are YES and NO.