patch 8.1.0783: compiler warning for signed/unsigned

Problem:    Compiler warning for signed/unsigned.
Solution:   Add type cast.  Change type of buffer. (Ozaki Kiichi, closes #3827)
diff --git a/src/main.c b/src/main.c
index 18be06c..2fafe94 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1288,7 +1288,7 @@
 		 * string here. Don't reset keep_msg, msg_attr_keep() uses it
 		 * to check for duplicates. */
 		p = keep_msg;
-		msg_attr(p, keep_msg_attr);
+		msg_attr((char *)p, keep_msg_attr);
 		vim_free(p);
 	    }
 	    if (need_fileinfo)		/* show file info after redraw */