c6x: kernel: setup: Add screen_info global variable

Or can not pass building with allmodconfig:

LD init/built-in.o
drivers/built-in.o: In function `vgacon_switch':
vgacon.c:(.text+0x47f8): undefined reference to `screen_info'
vgacon.c:(.text+0x4810): undefined reference to `screen_info'
drivers/built-in.o: In function `vgacon_resize':
vgacon.c:(.text+0x4ac8): undefined reference to `screen_info'
vgacon.c:(.text+0x4acc): undefined reference to `screen_info'
drivers/built-in.o: In function `vgacon_save_screen':
vgacon.c:(.text+0x4cc8): undefined reference to `screen_info'
drivers/built-in.o:vgacon.c:(.text+0x4cd0): more undefined references to `screen_info' follow

Signed-off-by: Chen Gang <[e-mail]>
Signed-off-by: Mark Salter <[e-mail]>

arch/c6x/kernel/setup.c

···
 #include <linux/fs.h>
 #include <linux/of.h>
 #include <linux/console.h>
+#include <linux/screen_info.h>
 
 #include <asm/sections.h>
 #include <asm/div64.h>
···
 #include <asm/special_insns.h>
 
 static const char *c6x_soc_name;
+
+struct screen_info screen_info;
 
 int c6x_num_cores;
 EXPORT_SYMBOL_GPL(c6x_num_cores);