Add missing header 'arithmetic.hh' to Makefile.am

[[Imported from SVN: r12941]]

dune/solvers/common/Makefile.am

 SUBDIRS =
 
 commondir = $(includedir)/dune/solvers/common
-common_HEADERS = boxconstraint.hh \
+common_HEADERS = arithmetic.hh \
+                 boxconstraint.hh \
                  genericvectortools.hh \
                  canignore.hh \
                  interval.hh \