alpha: mmu_context: Add missing include of linux/sched.h

Fix the following build error.

In file included from include/linux/mmu_context.h:4:0,
from drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd.h:29, from drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd.c:23:
arch/alpha/include/asm/mmu_context.h: In function 'ev5_switch_mm':
arch/alpha/include/asm/mmu_context.h:160:2: error:
implicit declaration of function 'task_thread_info'

---
"Exposed-by:" instead of "Fixes:" might be more appropriate.

 arch/alpha/include/asm/mmu_context.h | 1 +
 1 file changed, 1 insertion(+)

index 384bd47b5187..45c020a0fe76 100644
@@ -8,6 +8,7 @@
  */
 
 #include <linux/mm_types.h>
+#include <linux/sched.h>
 
 #include <asm/machvec.h>
 #include <asm/compiler.h>

2.7.4