IB/qib: Add missing <linux/slab.h> include

Fix build failure on sparc64 which is missing the include of
<linux/slab.h> via <asm/pci.h> that x86, powerpc, ia64, etc. have.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Roland Dreier <rolandd@cisco.com>