powerpc/powermac: Move pmac_pfunc_base_install prototype to header file

The pmac_pfunc_base_install prototype was declared in powermac/smp.c since
function was used there, move it to pmac_pfunc.h header to be visible in
pfunc_base.c. Fix a warning treated as error with W=1:

  arch/powerpc/platforms/powermac/pfunc_base.c:330:12: error: no previous prototype for ‘pmac_pfunc_base_install’ [-Werror=missing-prototypes]

Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>