render: miindex.c does not need header guard macros

Clears warning from clang 19.1.7:

render/miindex.c:27:9: warning: macro is not used [-Wunused-macros]
   27 | #define _MIINDEX_H_
      |         ^

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1910>

render/miindex.c

@@ -23,9 +23,6 @@
 
 #include <dix-config.h>
 
-#ifndef _MIINDEX_H_
-#define _MIINDEX_H_
-
 #include "dix/colormap_priv.h"
 
 #include "scrnintstr.h"
@@ -328,5 +325,3 @@ miUpdateIndexed(ScreenPtr pScreen,
         }
     }
 }
-
-#endif                          /* _MIINDEX_H_ */