fbcon: mark dummy functions 'inline'

The newly introduced fb_console_init/exit declarations have a
dummy alternative that is marked 'static' in the header but not
inline, leading to a warning whenever the header gets included:

In file included from drivers/video/fbdev/core/fbmem.c:35:0:
include/linux/fbcon.h:9:13: error: 'fb_console_exit' defined but not used [-Werror=unused-function]

This adds the intended 'inline'.

Fixes: 6104c370 ("fbcon: Make fbcon a built-time depency for fbdev")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>