patch 8.2.1597: the channel source file is too big

Problem:    The channel source file is too big.
Solution:   Move job related code to a new source file.
diff --git a/src/proto.h b/src/proto.h
index e715ca4..5923d23 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -278,6 +278,7 @@
 #  include "netbeans.pro"
 # endif
 # ifdef FEAT_JOB_CHANNEL
+#  include "job.pro"
 #  include "channel.pro"
 
 // Not generated automatically, to add extra attribute.