fbcon: Kill compile warning

Fix compile warning ('map_override unused') if fbcon is compiled as a module
and CONFIG_FRAMEBUFFER_CONSOLE_DETECT_PRIMARY=n.

[akpm@linux-foundation.org: cleanup]
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>