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.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/sysrq.c

@@ -74,6 +74,7 @@ int sysrq_mask(void)
 		return 1;
 	return sysrq_enabled;
 }
+EXPORT_SYMBOL_GPL(sysrq_mask);
 
 /*
  * A value of 1 means 'all', other nonzero values are an op mask:
@@ -1058,6 +1059,7 @@ int sysrq_toggle_support(int enable_mask)
 
 	return 0;
 }
+EXPORT_SYMBOL_GPL(sysrq_toggle_support);
 
 static int __sysrq_swap_key_ops(int key, struct sysrq_key_op *insert_op_p,
                                 struct sysrq_key_op *remove_op_p)