Remove the wrong "std::" scope prefix before ceil() call.

Remove the wrong "std::" scope prefix before ceil() call.

Fix compilation errors with VC6 (and possibly other compilers) introduced in
r70797 (see #13383): as we only include <math.h> and not <cmath> we don't
necessarily have std::ceil() available, only the extern "C" ceil(). Just use
the latter instead of the former.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70811 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775