tty/sysrq: Export sysrq_mask(), sysrq_toggle_support()

Build fix for serial_core being module:
  ERROR: modpost: "sysrq_toggle_support" [drivers/tty/serial/serial_core.ko] undefined!
  ERROR: modpost: "sysrq_mask" [drivers/tty/serial/serial_core.ko] undefined!

Fixes: eaee4172

 ("sysctl/sysrq: Remove __sysrq_enabled copy")
Cc: Jiri Slaby <jslaby@suse.com>
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20200420172317.599611-1-dima@arista.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>