net/octeontx: fix meson build for disabled drivers

net/octeontx: fix meson build for disabled drivers

Add a condition to check if octeontx drivers are disabled.
octeontx drivers are built only if dependent drivers i.e.
ethdev, mempool and common/octeontx are enabled.

Bugzilla ID: 387
Fixes: 7f615033d64f ("drivers/net: build Cavium NIC PMDs with meson")
Cc: stable@dpdk.org
Signed-off-by: Amit Gupta <agupta3@marvell.com>
Reviewed-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Harman Kalra <hkalra@marvell.com>