cx25821-alsa: shutup a Gcc 6.1 warning

The PCI device ID table is only used if compiled with modules
support. When compiled with modules disabled, this is now
producing this bogus warning:

drivers/media/pci/cx25821/cx25821-alsa.c:696:35: warning: 'cx25821_audio_pci_tbl' defined but not used [-Wunused-const-variable=]
 static const struct pci_device_id cx25821_audio_pci_tbl[] = {
                                   ^~~~~~~~~~~~~~~~~~~~~

Fix it by annotating that the function may not be used.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>