gbefb: fix compile error

The patch "drivers/video/gbefb.c: use devm_ functions" caused a
compile error.

drivers/video/gbefb.c:1159:16: error: implicit declaration of function
'devm_ioremap' [-Werror=implicit-function-declaration]
drivers/video/gbefb.c:1179:3: error: implicit declaration of function
'devm_ioremap_nocache' [-Werror=implicit-function-declaration]

Fix it by including linux/io.h which defines those functions.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Damien Cassou <damien.cassou@lifl.fr>