Add include dependencies to <linux/printk.h>.

If <linux/linkage.h> has not been included before <linux/printk.h>,
a build error like the below one will result:

  CC      arch/mips/kernel/idle.o
In file included from arch/mips/kernel/idle.c:17:0:
include/linux/printk.h:109:1: error: data definition has no type or storage class [-Werror]
include/linux/printk.h:109:1: error: type defaults to ‘int’ in declaration of ‘asmlinkage’ [-Werror=implicit-int]
include/linux/printk.h:110:1: error: ‘format’ attribute only applies to function types [-Werror=attributes]
include/linux/printk.h:110:1: error: expected ‘,’ or ‘;’ before ‘int’
include/linux/printk.h:114:1: error: data definition has no type or storage class [-Werror]
include/linux/printk.h:114:1: error: type defaults to ‘int’ in declaration of ‘asmlinkage’ [-Werror=implicit-int]
include/linux/printk.h:115:1: error: ‘format’ attribute only applies to function types [-Werror=attributes]
include/linux/printk.h:115:1: error: expected ‘,’ or ‘;’ before ‘int’
include/linux/printk.h:117:1: error: data definition has no type or storage class [-Werror]
include/linux/printk.h:117:1: error: type defaults to ‘int’ in declaration of ‘asmlinkage’ [-Werror=implicit-int]
include/linux/printk.h:118:1: error: ‘format’ attribute only applies to function types [-Werror=attributes]
include/linux/printk.h:118:1: error: ‘__cold__’ attribute ignored [-Werror=attributes]
include/linux/printk.h:118:1: error: expected ‘,’ or ‘;’ before ‘asmlinkage’
include/linux/printk.h:122:1: error: data definition has no type or storage class [-Werror]
include/linux/printk.h:122:1: error: type defaults to ‘int’ in declaration of ‘asmlinkage’ [-Werror=implicit-int]
include/linux/printk.h:123:1: error: ‘format’ attribute only applies to function types [-Werror=attributes]
include/linux/printk.h:123:1: error: ‘__cold__’ attribute ignored [-Werror=attributes]
include/linux/printk.h:123:1: error: expected ‘,’ or ‘;’ before ‘int’
In file included from include/linux/kernel.h:14:0,
                 from include/linux/sched.h:15,
                 from arch/mips/kernel/idle.c:18:
include/linux/dynamic_debug.h: In function ‘ddebug_dyndbg_module_param_cb’:
include/linux/dynamic_debug.h:124:3: error: implicit declaration of function ‘printk’ [-Werror=implicit-function-declaration]

Fixed by including <linux/linkage.h>.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>