patch 9.1.0432: Ancient XPM preprocessor hack may cause build errors

Problem:  Ancient XPM preprocessor hack may cause build errors.
Solution: Simplify XPM includes and get rid of complicated #ifdef magic
          (Drew Vogel).

closes: #14816

Signed-off-by: Drew Vogel <dvogel@github>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/vim16x16.xpm b/runtime/vim16x16.xpm
index cb75b71..9b55218 100644
--- a/runtime/vim16x16.xpm
+++ b/runtime/vim16x16.xpm
@@ -1,5 +1,5 @@
 /* XPM */
-static char * vim16x16[] = {
+static const char * vim16x16[] = {
 "16 16 8 1",
 " 	c None",
 ".	c #000000",