PCI: add declaration for pcibios_set_master() to pci core

Currently, pcibios_set_master() is implemented in architecture-
specific code.  There is nothing architecture-specific about PCI's
'latency timer'.

This patch adds a declaration for pcibios_set_master() to PCI's core
in preperation for pulling the function itself up into the core.
Without the addition of this declaration, subsequent patches that
remove inline definitions of pcibios_set_master() would be removing
the only declaration of such.

No functional change.

Signed-off-by: Myron Stowe <myron.stowe@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>