uml: Fix build breakage after slab.h changes

We now have to to include linux/slab.h explicitly for kmalloc &
friends.  Files that build against host headers already get their
prototypes via um_malloc.h, linux/slab.h may even be unavailable.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/arch/um/drivers/line.c b/arch/um/drivers/line.c
index 64cda95..7a656bd 100644
--- a/arch/um/drivers/line.c
+++ b/arch/um/drivers/line.c
@@ -6,6 +6,7 @@
 #include "linux/irqreturn.h"
 #include "linux/kd.h"
 #include "linux/sched.h"
+#include "linux/slab.h"
 #include "chan_kern.h"
 #include "irq_kern.h"
 #include "irq_user.h"