fbmem: Mark proc_fb_seq_ops as __maybe_unused

With CONFIG_PROC_FS=n and -Werror, 0-day reports:

drivers/video/fbdev/core/fbmem.c:736:36: error:
	'proc_fb_seq_ops' defined but not used

Mark it as __maybe_unused.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210504142910.2084722-1-linux@roeck-us.net