The function with the same name is defined in multiple libraries
(libfreerdp-shadow.so, libfreerdp-server.so), which might confuse
the dynamic linker at runtime, binding a wrong version of the function
and thus causing segmentation faults.
Since remdesk_virtual_channel_write()s aren't used outside the files
they are defined in, we can declare them static to make them invisible
to the linker.
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