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

drivers/video/fbdev/core/fbmem.c

@@ -733,7 +733,7 @@ static int fb_seq_show(struct seq_file *m, void *v)
 	return 0;
 }
 
-static const struct seq_operations proc_fb_seq_ops = {
+static const struct __maybe_unused seq_operations proc_fb_seq_ops = {
 	.start	= fb_seq_start,
 	.next	= fb_seq_next,
 	.stop	= fb_seq_stop,