Suppress a bogus -Warray-bounds in modules/superres/src/optical_flow.cpp

This is actually the same warning that the suppression at the bottom
suppresses, but that one doesn't suppress it when using a native compiler
(go figure).

I remove the forced inclusion of precompiled headers into all sources,
since otherwise this workaround doesn't work - the header with the warning
is included before I can use a pragma.
Merge pull request #3401 from SpecLad:optflow-bogus-Warray-bounds