Suppress various harmless warnings in MinGW build with -Wconversion.

No real changes, simply add casts to make the implicit conversions that g++
warns about when using -Wconversion explicit.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67964 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/mediactrl_am.cpp

@@ -2020,7 +2020,7 @@ wxLongLong wxAMMediaBackend::GetDuration()
 
         case S_OK:
             // outDuration is in seconds, we need milliseconds
-            return outDuration * 1000;
+            return static_cast<wxLongLong>(outDuration * 1000);
     }
 }