riscv: mmiowb: Fix implicit declaration of function 'smp_processor_id'

In file included from ./../include/linux/compiler_types.h:68,
                 from <command-line>:
../include/asm-generic/mmiowb.h: In function ‘mmiowb_set_pending’:
../include/asm-generic/percpu.h:34:38: error: implicit declaration of function ‘smp_processor_id’; did you mean ‘raw_smp_processor_id’? [-Werror=implicit-function-declaration]
 #define my_cpu_offset per_cpu_offset(smp_processor_id())
                                      ^~~~~~~~~~~~~~~~
../include/linux/compiler-gcc.h:58:26: note: in definition of macro ‘RELOC_HIDE’
  (typeof(ptr)) (__ptr + (off));     \
                          ^~~
../include/linux/percpu-defs.h:249:2: note: in expansion of macro ‘SHIFT_PERCPU_PTR’
  SHIFT_PERCPU_PTR(ptr, my_cpu_offset);    \
  ^~~~~~~~~~~~~~~~
../include/asm-generic/percpu.h:34:23: note: in expansion of macro ‘per_cpu_offset’
 #define my_cpu_offset per_cpu_offset(smp_processor_id())
                       ^~~~~~~~~~~~~~
../include/linux/percpu-defs.h:249:24: note: in expansion of macro ‘my_cpu_offset’
  SHIFT_PERCPU_PTR(ptr, my_cpu_offset);    \
                        ^~~~~~~~~~~~~
../include/asm-generic/mmiowb.h:30:26: note: in expansion of macro ‘this_cpu_ptr’
 #define __mmiowb_state() this_cpu_ptr(&__mmiowb_state)
                          ^~~~~~~~~~~~
../include/asm-generic/mmiowb.h:37:28: note: in expansion of macro ‘__mmiowb_state’
  struct mmiowb_state *ms = __mmiowb_state();
                            ^~~~~~~~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/mmiowb.h

@@ -9,6 +9,7 @@
  */
 #define mmiowb()	__asm__ __volatile__ ("fence o,w" : : : "memory");
 
+#include <linux/smp.h>
 #include <asm-generic/mmiowb.h>
 
 #endif	/* _ASM_RISCV_MMIOWB_H */