VPP-31 Fix linker issue with new sr-replicate

VPP-31 Fix linker issue with new sr-replicate

Changes in gnu ld in Ubuntu 16.04 tickles an issue with the
declaration pattern used for vlib_node_registration when,
as is the case in sr_replicate.c, the forward declaration is
seen by the linker in a module before the real declaration.

This patch uses the "extern" keyword in the forward declaration
to avoid this issue and removes extra forward declarations.

Change-Id: I075b0cd9932404e1360c7f9aec7114003d110f60
Signed-off-by: Chris Luke <chrisy@flirble.org>