[PATCH v2] asm-generic: barrier.h: fix ALPHA builds when SMP is not enabled

[Randy Dunlap]

<asm-generic/barrier.h> uses barrier() but needs to #include
<linux/compiler.h> to get a generic definition of it to fix
build errors in arch/alpha/ builds when CONFIG_SMP is not enabled.

Fixes build errors like this (there are several like this):
[49 in the kernel config that was supplied by the 0day bot]

In file included from ../arch/alpha/include/asm/barrier.h:21,
                 from ../arch/alpha/include/asm/atomic.h:6,
                 from ../include/linux/atomic.h:7,
                 from ../include/linux/dcache.h:5,
                 from ../fs/proc/util.c:1:
../arch/alpha/include/asm/atomic.h: In function 'atomic_add_return_relaxed':
../include/asm-generic/barrier.h:78:18: error: implicit declaration of function 'barrier' [-Werror=implicit-function-declaration]
   78 | #define smp_mb() barrier()
      |                  ^~~~~~~

Fixes: 885df91ca357 ("Create asm-generic/barrier.h")
Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Richard Henderson <rth@xxxxxxxxxxx>
Cc: Ivan Kokshaysky <ink@xxxxxxxxxxxxxxxxxxxx>
Cc: Matt Turner <mattst88@xxxxxxxxx>
Cc: linux-alpha@xxxxxxxxxxxxxxx
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
---
v2: also send to Andrew and Stephen.

 include/asm-generic/barrier.h |    1 +
 1 file changed, 1 insertion(+)

--- linux-next-20201030.orig/include/asm-generic/barrier.h
+++ linux-next-20201030/include/asm-generic/barrier.h
@@ -13,6 +13,7 @@
 
 #ifndef __ASSEMBLY__
 
+#include <linux/compiler.h>
 #include <asm/rwonce.h>
 
 #ifndef nop