patch 8.0.1827: compiler warning for signed/unsigned char pointers

Problem:    Compiler warning for signed/unsigned char pointers. (Cesar Romani)
Solution:   Change the type of jv_argv.
diff --git a/src/structs.h b/src/structs.h
index 5c4a74c..b70b00f 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1488,7 +1488,7 @@
     int		jv_copyID;
 
     channel_T	*jv_channel;	/* channel for I/O, reference counted */
-    char_u	**jv_argv;	/* command line used to start the job */
+    char	**jv_argv;	/* command line used to start the job */
 };
 
 /*