rpcrt4: Cast-qual warnings fix (1 of 2)

[Andrew Talbot]

Changelog:
    rpcrt4: Cast-qual warnings fix.

diff -urN a/dlls/rpcrt4/cstub.c b/dlls/rpcrt4/cstub.c
--- a/dlls/rpcrt4/cstub.c	2006-11-10 17:26:30.000000000 +0000
+++ b/dlls/rpcrt4/cstub.c	2006-11-11 12:33:38.000000000 +0000
@@ -37,7 +37,7 @@
 
 WINE_DEFAULT_DEBUG_CHANNEL(ole);
 
-#define STUB_HEADER(This) (((CInterfaceStubHeader*)((This)->lpVtbl))[-1])
+#define STUB_HEADER(This) (((const CInterfaceStubHeader*)((This)->lpVtbl))[-1])
 
 static WINE_EXCEPTION_FILTER(stub_filter)
 {