KVM: PPC: Book3S: Fix compile error in XICS emulation

Commit 8e44ddc3 ("powerpc/kvm/book3s: Add support for H_IPOLL and
H_XIRR_X in XICS emulation") added a call to get_tb() but didn't
include the header that defines it, and on some configs this means
book3s_xics.c fails to compile:

arch/powerpc/kvm/book3s_xics.c: In function ‘kvmppc_xics_hcall’:
arch/powerpc/kvm/book3s_xics.c:812:3: error: implicit declaration of function ‘get_tb’ [-Werror=implicit-function-declaration]

Cc: stable@vger.kernel.org [v3.10, v3.11]
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>