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/proto.h b/src/proto.h
index 10c8ff5..05a7cf8 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -112,6 +112,7 @@
 # ifdef FEAT_VIMINFO
 # include "viminfo.pro"
 # endif
+# include "evalvars.pro"
 
 /* These prototypes cannot be produced automatically. */
 int smsg(const char *, ...)