patch 8.1.1933: the eval.c file is too big

Problem:    The eval.c file is too big.
Solution:   Move code related to variables to evalvars.c. (Yegappan
            Lakshmanan, closes #4868)
diff --git a/src/Make_morph.mak b/src/Make_morph.mak
index 08111aa..432efa2 100644
--- a/src/Make_morph.mak
+++ b/src/Make_morph.mak
@@ -42,6 +42,7 @@
 	edit.c							\
 	eval.c							\
 	evalfunc.c						\
+	evalvars.c						\
 	ex_cmds.c						\
 	ex_cmds2.c						\
 	ex_docmd.c						\