riscv: syscall_table: Reduce W=1 compilation warnings noise

Building riscv syscall table with W=1 throws the warning as
following pattern:
arch/riscv/kernel/syscall_table.c:14:36: warning: initialized field overwritten [-Woverride-init]
   14 | #define __SYSCALL(nr, call) [nr] = (call),
      |                                    ^
./include/uapi/asm-generic/unistd.h:29:37: note: in expansion of macro ‘__SYSCALL’
   29 | #define __SC_COMP(_nr, _sys, _comp) __SYSCALL(_nr, _sys)
      |                                     ^~~~~~~~~
./include/uapi/asm-generic/unistd.h:34:1: note: in expansion of macro ‘__SC_COMP’
   34 | __SC_COMP(__NR_io_setup, sys_io_setup, compat_sys_io_setup)
      | ^~~~~~~~~
arch/riscv/kernel/syscall_table.c:14:36: note: (near initialization for ‘sys_call_table[0]’)
   14 | #define __SYSCALL(nr, call) [nr] = (call),
      |                                    ^
./include/uapi/asm-generic/unistd.h:29:37: note: in expansion of macro ‘__SYSCALL’
   29 | #define __SC_COMP(_nr, _sys, _comp) __SYSCALL(_nr, _sys)
      |                                     ^~~~~~~~~
./include/uapi/asm-generic/unistd.h:34:1: note: in expansion of macro ‘__SC_COMP’
   34 | __SC_COMP(__NR_io_setup, sys_io_setup, compat_sys_io_setup)
      | ^~~~~~~~~
arch/riscv/kernel/syscall_table.c:14:36: warning: initialized field overwritten [-Woverride-init]
...

Since we intentionally build the syscall tables this way,ignore the warning.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>