msdmo: Constify some variables

[Andrew Talbot]

Changelog:
    msdmo: Constify some variables.

diff -urN a/dlls/msdmo/dmoreg.c b/dlls/msdmo/dmoreg.c
--- a/dlls/msdmo/dmoreg.c	2007-01-12 11:40:50.000000000 +0000
+++ b/dlls/msdmo/dmoreg.c	2007-05-28 14:58:41.000000000 +0100
@@ -102,7 +102,7 @@
     return lpwstr;
 }
 
-static BOOL IsMediaTypeEqual(DMO_PARTIAL_MEDIATYPE* mt1, DMO_PARTIAL_MEDIATYPE* mt2)
+static BOOL IsMediaTypeEqual(const DMO_PARTIAL_MEDIATYPE* mt1, const DMO_PARTIAL_MEDIATYPE* mt2)
 {
 
     return (IsEqualCLSID(&mt1->type, &mt2->type) ||