console: fix section mismatch warning in vgacon.c

Fix following section mismatch warning:
WARNING: vmlinux.o(.text+0x121e62): Section mismatch: reference to .init.text:__alloc_bootmem (between 'vgacon_startup' and 'vgacon_scrolldelta')

Browsing the code it seems that vgacon_scrollback_startup() is only called
during the init phase so the reference to the .init.text section is OK.

Teach modpost not to warn using ___init_refok.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>