alpha: fix build breakage in asm/cacheflush.h

Alpha SMP flush_icache_user_range() is implemented as an inline
function inside include/asm/cacheflush.h.  It dereferences @current
but doesn't include linux/sched.h and thus causes build failure if
linux/sched.h wasn't included previously.  Fix it by including the
needed header file explicitly.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Matt Turner <mattst88@gmail.com>

arch/alpha/include/asm/cacheflush.h

@@ -43,6 +43,8 @@ extern void smp_imb(void);
 /* ??? Ought to use this in arch/alpha/kernel/signal.c too.  */
 
 #ifndef CONFIG_SMP
+#include <linux/sched.h>
+
 extern void __load_new_mm_context(struct mm_struct *);
 static inline void
 flush_icache_user_range(struct vm_area_struct *vma, struct page *page,