added missing header.

dune/fem-dg/test/euler/Makefile.am

 # install these headers 
 eulerdir=$(includedir)/dune/fem-dg/test/euler
 euler_HEADERS= eulermodel.hh  problems.hh problemcreator.hh \
-chorjo.hh chorjo.cc 							 
+chorjo.hh chorjo.cc passtraits.hh
 
 LDADD = $(ALL_PKG_LDFLAGS) $(ALL_PKG_LIBS) $(LOCAL_LIBS) $(DUNEMPILDFLAGS) $(DUNEMPILIBS)