patch 8.0.1775: MS-Windows: warning for unused variable

Problem:    MS-Windows: warning for unused variable.
Solution:   Move declaration inside #ifdef. (Mike Williams)
diff --git a/src/channel.c b/src/channel.c
index 2c82efa..67be1d7 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -5496,12 +5496,12 @@
     int		argc = 0;
 #if defined(UNIX)
 # define USE_ARGV
+    int		i;
 #else
     garray_T	ga;
 #endif
     jobopt_T	opt;
     ch_part_T	part;
-    int		i;
 
     job = job_alloc();
     if (job == NULL)