powerpc: Fix build by disabling attribute-alias warning for SYSCALL_DEFINEx

GCC 8.1 emits warnings such as the following. As arch/powerpc code is
built with -Werror, this breaks the build with GCC 8.1.

  In file included from arch/powerpc/kernel/pci_64.c:23:
  ./include/linux/syscalls.h:233:18: error: 'sys_pciconfig_iobase' alias
  between functions of incompatible types 'long int(long int, long
  unsigned int, long unsigned int)' and 'long int(long int, long int,
  long int)' [-Werror=attribute-alias]
    asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__)) \
                    ^~~
  ./include/linux/syscalls.h:222:2: note: in expansion of macro '__SYSCALL_DEFINEx'
    __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)

This patch inhibits those warnings.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Trim change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>