alpha: fix implicit stat.h usage in pci-sysfs.c

To avoid this:

arch/alpha/kernel/pci-sysfs.c:164: error: 'S_IRUSR' undeclared (first use in this function)
arch/alpha/kernel/pci-sysfs.c:164: error: 'S_IWUSR' undeclared (first use in this function)
make[2]: *** [arch/alpha/kernel/pci-sysfs.o] Error 1

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>