locking/qspinlock: Explicitly include asm/prefetch.h

In architectures that use qspinlock, like x86, prefetch is loaded
indirectly via the asm/qspinlock.h include.  On other architectures, like
OpenRISC, which may want to use asm-generic/qspinlock.h the built will
fail without the asm/prefetch.h include.

Fix this by including directly.
Signed-off-by: Stafford Horne <shorne@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170707195658.23840-1-shorne@gmail.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/qspinlock.c

@@ -28,6 +28,7 @@
 #include <linux/percpu.h>
 #include <linux/hardirq.h>
 #include <linux/mutex.h>
+#include <linux/prefetch.h>
 #include <asm/byteorder.h>
 #include <asm/qspinlock.h>