patch 8.0.1849: compiler warning for unused arguments, missing prototype

Problem:    compiler warning for unused arguments and missing prototype
Solution:   Add UNUSED.  Add static.
diff --git a/src/mbyte.c b/src/mbyte.c
index 6d21f11..b797835 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -6506,7 +6506,7 @@
 
 #  ifdef FEAT_GUI
     void
-im_set_position(int row, int col)
+im_set_position(int row UNUSED, int col UNUSED)
 {
 }
 #  endif