datapath: Add missing targets to avoid failure on e.g. "make TAGS".

datapath: Add missing targets to avoid failure on e.g. "make TAGS".

Automake invokes a number of targets recursively, including in
datapath/linux, so we need to define all those targets or get an error
from "make" when those targets are invoked, e.g. when "make TAGS" is run.
This commit adds a no-op target to the main datapath/linux Makefile for
each recursive target listed in the "Third-Party Makefiles" section of
the Automake manual, in the order listed there, fixing the problem.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>