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

Commit b02d0ed6 ('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: NBen Hutchings <ben@decadent.org.uk>
Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>,
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/xhci-pci.c

@@ -21,6 +21,7 @@
  */
 
 #include <linux/pci.h>
+#include <linux/slab.h>
 
 #include "xhci.h"