patch 8.0.1321: can't build huge version with Athena

Problem:    Can't build huge version with Athena. (Mark Kelly)
Solution:   Move including beval.h to before structs.h. Include beval.pro like
            other proto files.
diff --git a/src/proto.h b/src/proto.h
index a60d3c8..5513746 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -201,7 +201,9 @@
 
 /* Ugly solution for "BalloonEval" not being defined while it's used in some
  * .pro files. */
-# ifndef FEAT_BEVAL
+# ifdef FEAT_BEVAL
+#  include "beval.pro"
+# else
 #  define BalloonEval int
 # endif