xhci-hcd: Include <linux/slab.h> in xhci-pci.c

Commit b02d0ed677acb3465e7600366f2353413bf24074 ('xhci: Change
hcd_priv into a pointer') added calls to kzalloc() and kfree() in
xhci-pci.c.  On most architectures <linux/slab.h> is indirectly
included, but on some it is not.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>,
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>