fbdev: fbmem: avoid exporting fb_center_logo

The variable is only ever used from fbcon.c which is linked into the
same module. Therefore, the export is not needed.

Signed-off-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190827110854.12574-4-peda@axentia.se