loader: Make vidc_biosputchar static

It's currently unused outside of vidconsole.c. Gerald Hicks' fix to the
beep code from de37e4a6d2333/1998 introduced the funciton as
static. Maxim Sobolev (sobomax) made it non-static since his spinconsole
called it in c4c3b35172d67/2009. When sobomax dropped the direct call
after making spinconsole console independent in b35172d67/2017,
vidc_biosputchar remained a harmless unreferenced global. Make it static
once again.

Fixes: c7e10205ae0d
Sponsored by: Netflix