patch 8.2.0872: XIM code is mixed with multi-byte code

Problem:    XIM code is mixed with multi-byte code.
Solution:   Move the XIM code to a separate file. (Yegappan Lakshmanan,
            closes #6177)
diff --git a/src/proto.h b/src/proto.h
index b73350a..eec7ad5 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -92,6 +92,7 @@
 # include "findfile.pro"
 # include "fold.pro"
 # include "getchar.pro"
+# include "gui_xim.pro"
 # include "hardcopy.pro"
 # include "hashtab.pro"
 # include "highlight.pro"