powerpc: fix build failure by disabling attribute-alias warning in pci_32

Commit 2479bfc9

 ("powerpc: Fix build by disabling attribute-alias
warning for SYSCALL_DEFINEx") forgot arch/powerpc/kernel/pci_32.c

Latest GCC version emit the following warnings

As arch/powerpc code is built with -Werror, this breaks build with
GCC 8.1

This patch inhibits this warning

In file included from arch/powerpc/kernel/pci_32.c:14:
./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__)
  ^~~~~~~~~~~~~~~~~

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