Fixup Makefile for PR#242 "Make exprt::is_zero() support bitfields"

Author: chrisr-diffblue

unit/Makefile

@@ -37,6 +37,7 @@ SRC += unit_tests.cpp \
        solvers/refinement/string_refinement/substitute_array_list.cpp \
        solvers/refinement/string_refinement/sparse_array.cpp \
        solvers/refinement/string_refinement/union_find_replace.cpp \
+       util/expr.cpp \
        util/expr_cast/expr_cast.cpp \
        util/graph.cpp \
        util/irep.cpp \