testsuite: declare dependency between $(TESTS) and generate_nlmsg

testsuite: declare dependency between $(TESTS) and generate_nlmsg

Parallel make from the top level directory fails since tests are at the
same time as generate_nlmsg:

$ make check -j4

...

cd testsuite && make && make alltests
echo "Entering iproute2" && cd iproute2 && make configure && cd ..;
Entering iproute2
make -C tools
Removing results dir ...
make[1]: ./tools/generate_nlmsg: Command not found
make[1]: ./tools/generate_nlmsg: Command not found
Makefile:64: recipe for target 'ip/netns/set_nsid_batch.t' failed
make[1]: *** [ip/netns/set_nsid_batch.t] Error 127
make[1]: ./tools/generate_nlmsg: Command not found
make[1]: *** Waiting for unfinished jobs....
Makefile:64: recipe for target 'ip/netns/set_nsid.t' failed
make[1]: *** [ip/netns/set_nsid.t] Error 127
Makefile:64: recipe for target 'ip/link/show_dev_wo_vf_rate.t' failed
make[1]: *** [ip/link/show_dev_wo_vf_rate.t] Error 127
    CC       generate_nlmsg
Makefile:123: recipe for target 'check' failed
make: *** [check] Error 2

Add an explicit dependency in testuite/Makefile's $(TESTS) rule so
that the tool correctly gets compiled before any test runs.

Fixes: 3537633dcf44 ("testsuite: Generate generate_nlmsg when needed")
Signed-off-by: Luca Boccassi <bluca@debian.org>
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>
Tested-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>