mk: do not install meson.build in usertools

In commit 7dd34c71de2a ("usertools: install with meson") meson.build was
added in usertools directory and so it's copied to $(datadir)/usertools
with "make install".

This patch avoids to copy meson.build when installing usertools with
"make install".

Fixes: 7dd34c71de2a ("usertools: install with meson")
Cc: stable@dpdk.org
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Acked-by: Luca Boccassi <bluca@debian.org>