added missing return to avoid 'control reaches end of non-void function'

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@16206 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/unix/utilsunx.cpp

@@ -844,6 +844,8 @@ long wxExecute(wxChar **argv,
         return exitcode;
 #endif // wxUSE_GUI
     }
+
+    return ERROR_RETURN_CODE;
 }
 
 #undef ERROR_RETURN_CODE