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

riscv's <vdso/processor.h> uses barrier() so it should include
<asm/barrier.h>

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 allmodconfig now builds cleanly after this patch.

Fixes fallout from:
815f0ddb34 ("include/linux/compiler*.h: make compiler-*.h mutually exclusive")

Fixes: ad5d1122b8 ("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>
Acked-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

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