powerpc/vdso: Add missing includes and clean vdso_setup_syscall_map()

Instead of including extern references locally in
vdso_setup_syscall_map(), add the missing headers.

sys_ni_syscall() being a function, cast its address to
an unsigned long instead of declaring it as a fake
unsigned long object.

At the same time, remove a comment which paraphrases the
function name.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b4afedce748ed2858299ceab5ae29b52109263ef.1601197618.git.christophe.leroy@csgroup.eu