Add semaphore.h to kernel_lock.c

kernel_lock.c uses DECLARE_MUTEX, up() and down() without explicitly
including asm/semaphore.h. This is fragile and leaves it vulnerable
to breakage during header reorganisations.

Signed-off-by: Matthew Wilcox <[e-mail]>

lib/kernel_lock.c

···
 #include <linux/smp_lock.h>
 #include <linux/module.h>
 #include <linux/kallsyms.h>
+#include <asm/semaphore.h>
 
 /*
  * The 'big kernel semaphore'