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>

drivers/video/gbefb.c

@@ -20,6 +20,7 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/module.h>
+#include <linux/io.h>
 
 #ifdef CONFIG_X86
 #include <asm/mtrr.h>
@@ -28,7 +29,6 @@
 #include <asm/addrspace.h>
 #endif
 #include <asm/byteorder.h>
-#include <asm/io.h>
 #include <asm/tlbflush.h>
 
 #include <video/gbe.h>