Fix a build warning in viafbdev.c when CONFIG_PROC_FS is not enabled
by marking the unused function as __maybe_unused.

../drivers/video/fbdev/via/viafbdev.c:1471:12: warning: 'viafb_sup_odev_proc_show' defined but not used [-Wunused-function]
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>