oleaut32: Constify input parameter to VarR8FromDec.

With thanks to Dmitry for spotting this.

include/oleauto.h

diff --git a/include/oleauto.h b/include/oleauto.h
index 4492d04..98e8dbf 100644
--- a/include/oleauto.h
+++ b/include/oleauto.h
@@ -316,7 +316,7 @@
 HRESULT WINAPI VarR8FromUI8(ULONG64,double*);
 HRESULT WINAPI VarR8FromStr(OLECHAR*,LCID,ULONG,double*);
 HRESULT WINAPI VarR8FromCy(CY,double*);
-HRESULT WINAPI VarR8FromDec(DECIMAL*,double*);
+HRESULT WINAPI VarR8FromDec(const DECIMAL*,double*);
 HRESULT WINAPI VarR8FromDisp(IDispatch*,LCID,double*);
 
 HRESULT WINAPI VarDateFromUI1(BYTE,DATE*);