arm64: cmpxchg: Include linux/compiler.h in asm/cmpxchg.h

We need linux/compiler.h for unreachable(), so #include it here.
Reported-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/include/asm/cmpxchg.h

@@ -19,6 +19,7 @@
 #define __ASM_CMPXCHG_H
 
 #include <linux/build_bug.h>
+#include <linux/compiler.h>
 
 #include <asm/atomic.h>
 #include <asm/barrier.h>