dpdk: fix build error when mlx support enabled

dpdk: fix build error when mlx support enabled

When building with environment variables set to enable mlx PMD
support in DPDK, an error occurs:

CMake Error at plugins/dpdk/CMakeLists.txt:104 (vpp_plugin_find_library):
  vpp_plugin_find_library Macro invoked with incorrect arguments for macro
  named: vpp_plugin_find_library

Update a call to vpp_plugin_find_library() to include the right
number of parameters.

Change-Id: Ia0d66f93c6f94fdf822e2c3c4fe3f0ad01a90d57
Signed-off-by: Matthew Smith <mgsmith@netgate.com>