oleaut32: Constify some variables

[Andrew Talbot]

Changelog:
    oleaut32: Constify some variables.

diff -urN a/dlls/oleaut32/ungif.c b/dlls/oleaut32/ungif.c
--- a/dlls/oleaut32/ungif.c	2007-05-18 14:05:29.000000000 +0100
+++ b/dlls/oleaut32/ungif.c	2007-07-31 21:10:27.000000000 +0100
@@ -192,7 +192,7 @@
 static int
 AddExtensionBlock(SavedImage * New,
                   int Len,
-                  unsigned char ExtData[]) {
+                  const unsigned char ExtData[]) {
 
     ExtensionBlock *ep;
 
@@ -723,7 +723,7 @@
  * the maximum possible if image O.k. - LZ_MAX_CODE times.
  *****************************************************************************/
 static int
-DGifGetPrefixChar(GifPrefixType *Prefix,
+DGifGetPrefixChar(const GifPrefixType *Prefix,
                   int Code,
                   int ClearCode) {