sh: Fix vsyscall build failure.

  CC      arch/sh/kernel/vsyscall/vsyscall.o
a/arch/sh/kernel/vsyscall/vsyscall.c: In function 'arch_setup_additional_pages':
a/arch/sh/kernel/vsyscall/vsyscall.c:63: error: dereferencing pointer to incomplete type
a/arch/sh/kernel/vsyscall/vsyscall.c:67: error: dereferencing pointer to incomplete type
a/arch/sh/kernel/vsyscall/vsyscall.c:82: error: dereferencing pointer to incomplete type
a/arch/sh/kernel/vsyscall/vsyscall.c:85: error: dereferencing pointer to incomplete type
a/arch/sh/kernel/vsyscall/vsyscall.c: In function 'arch_vma_name':
a/arch/sh/kernel/vsyscall/vsyscall.c:91: error: dereferencing pointer to incomplete type

Signed-off-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>