output: add void argument to output_pre_cb_flush()
output: add void argument to output_pre_cb_flush()

Since co_create expects a function in the form of "void (*func)(void)",
we need an explicit void argument here.

Otherwise, MSVC complains with the following warning:

    warning C4113: 'void (__cdecl *)()' differs in parameter
    lists from 'void (__cdecl *)(void)'

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@clear-code.com>

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