drivers: tty: Mark the function hvc_poll_init() as static in hvc_console.c

Mark the function hvc_poll_init() as static in hvc/hvc_console.c because
it is not used outside this file.

This eliminates the following warning in hvc/hvc_console.c:
drivers/tty/hvc/hvc_console.c:791:5: warning: no previous prototype for ‘hvc_poll_init’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>