Warning fixes found under hardest mode of OpenWatcom. Seems clean in Borland, MinGW and DMC.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@29682 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/dc.cpp

@@ -2461,6 +2461,7 @@ static bool AlphaBlt(HDC hdcDst,
 #else // !wxHAVE_RAW_BITMAP
     // no wxAlphaBlend() neither, fall back to using simple BitBlt() (we lose
     // alpha but at least something will be shown like this)
+    wxUnusedVar(bmp);
     return false;
 #endif // wxHAVE_RAW_BITMAP
 }