[PATCH v2] RISC-V: fix barrier() use in <vdso/processor.h>

[Randy Dunlap <rdunlap@infradead.org>]

riscv's <vdso/processor.h> uses barrier() so it should
#include <asm/barrier.h> to prevent build errors.

Fixes this build error:
  CC [M]  drivers/net/ethernet/emulex/benet/be_main.o
In file included from ./include/vdso/processor.h:10,
                 from ./arch/riscv/include/asm/processor.h:11,
                 from ./include/linux/prefetch.h:15,
                 from drivers/net/ethernet/emulex/benet/be_main.c:14:
./arch/riscv/include/asm/vdso/processor.h: In function 'cpu_relax':
./arch/riscv/include/asm/vdso/processor.h:14:2: error: implicit declaration of function 'barrier' [-Werror=implicit-function-declaration]
   14 |  barrier();

This happens with a total of 5 networking drivers -- they all use
<linux/prefetch.h>.

rv64 and rv32 allmodconfig now build cleanly after this patch.

Fixes: 3347acc6fcd4 ("compiler.h: fix barrier_data() on clang")
Fixes: ad5d1122b82f ("riscv: use vDSO common flow to reduce the latency of the time-related functions")
Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: linux-riscv@lists.infradead.org
Cc: clang-built-linux@googlegroups.com
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
---
v2: update Fixes: tags (Nick);
    also built allmodconfig cleanly on rv32;

 arch/riscv/include/asm/vdso/processor.h |    2 ++
 1 file changed, 2 insertions(+)

--- lnx-510-rc4.orig/arch/riscv/include/asm/vdso/processor.h
+++ lnx-510-rc4/arch/riscv/include/asm/vdso/processor.h
@@ -4,6 +4,8 @@
 
 #ifndef __ASSEMBLY__
 
+#include <asm/barrier.h>
+
 static inline void cpu_relax(void)
 {
 #ifdef __riscv_muldiv