move pcie_bifurcation_lib to be static

The library won't be used outside of the pcie_bifurcation executable.
Making it static so that we don't need to install it as a separate
shared lib on the machine.

Google-Bug-Id: 281642829
Change-Id: Idcd1848de10442d36b90725c626106f5862d09ab
Signed-off-by: Willy Tu <wltu@google.com>